Seiko 5 sports srpd51 or if you can still find a seiko sea ui snzf17. Bother have an exhibition caseback.
Also, the ray 2 has better lume than the mako.
You could ask in r/Chinesewatches. In that price range there are plenty submariner homages like the SteelDive sd1975, maybe some have a display caseback.
You want a mechanical/automatic watch or an analog watch?
An analog quartz or solar would be more durable for whatever sportier activities kids do these days. They are also much more accurate than mechanical watches.
For automatics that are Japanese the citizen Promaster ny0086 would be one of the more robust ones.
Also the orient ray 2 or the seiko 5 sports models.
The Orient maestro is a modern/casual/minimalist design.
For analog quartz watches you have Casio that's Japanese with models like the mtd1053 (40mm), efv100d, efrs108, efrs107d, mts100 and mts110 (these have a sapphire crystal and a 10 year battery).
There are plenty of watches of Chinese or Hong Kong origin (like perpetual Watches) if he's from that part of the world, you can ask for recommendations in the r/Chinesewatches subredit.
The citizen eco-drive bm8180 would be a good choice, it's solar powered and small in size to fit a small wrist. It's also robust and would do well with all sorts of sports. Something like this would be my choice for a more classic looking watch that would keep up with an active lifestyle. It's also Japanese in origin like seiko, Casio, orient.
There are also many microbrands based in Asia like vario, zelos, Phoibos...
You could ask in r/watchrepair
Some limited edition seikos do, but it all.
That MK is a big watch even for a man at 43mm, does she want a big watch like that or that's just something she didn't think about.
Here are some smaller but similar watches in design.
Citizen FE7053-51X
Citizen FD2013-50A
You could browse here for something else in you are in the US. These are less expensive and better watches. Just don't hide it in a drawer in the dark when not wearing it so it can get some daylight and it'll just work for many years since it's solar.
If she actually wants something bigger, just browse the men's watches on the same store maybe there is something in roze gold.
If she can find something she likes citizen eco-drives are much better watches than MK. Something from Casio's Sheen line would also be a better choice when it comes to quality and durability but it's what she likes that's more important probably. Also a chronograph it's not the best choose unless she will actually use it to time things,if a watch is more complex there are more things that could go wrong with it.
There are some smaller divers from companies like San Martin (retro diver or retro gmt) or the seestern 62MAS homage (37mm). You could ask in r/ Chinesewatches too or just search on youtube for 37mm diver or 38mm dive watch or something similar.
If you are in the US get an eco-drive you feel would fit him from this ebay store. They have the best prices. At the moment they do t have the most stock but there are enough to pick one.
Here's one that might fit your description.
An all stainless steel (without gold colored plating) would probably fair better over time like this one (this model has a fixed bezel, there are other models with a similar (diver) style that have a rotating bezel that can be used as a 60 minutes timer like this one - o The larger side). The dive style of watches is the most popular stye of watches among men.
There's also an extra 25% off coupon at the moment for some models including both of the ones I mentioned.
Eco-drives are very good practical everyday watches. They are durable, accurate and don't require any sort of mentenance, just don't hide it in a drawer in the dark when not wearing it so it can get some daylight and it'll just work for many years.
Keep in mind the size also. Usually around 40mm is ok for most wrists for a watch without an external bezel and around 42mm would be a good size for a dive style watch. Depending on how big his wrist is you can go higher or lower than these measurements.
Also, titanium watches like the o e I mentioned are much lighter than the stainless steel ones, some people like that, others don't.
For mechanical something with the seagull st19 movement. Sugess have some more interesting designs like the heritage chrono.
The solar orient neo 70s panda is pretty cool.
Baltany s5050 tuxedo or the Dan henry 1939 or 1937 or 1964 or 1954 for something vintage looking or the top gun /maverick one.
Seiko flightmaster sna411 or the citizen bullhead reissue for something a little different.
There are a lot of microbrands that use the seiko meca-quartz movements.
You could also look on sakura watches for soml jdm seiko chronos.
You could also check out birthyearwatches.com. You might find something interesting if you're lucky. Usually they have a video with the watches they sell.
If you are in the US take a look at the officialwatchdeals store on ebay and get a citizen eco-drive you like from there. It's probably the best watch you can find. They don't have much stock at the moment since it's before Christmas but there are still plenty to choose from. Keep in mind the size of you have a small wrist.
This BM6929-56L would be a great everyday watch. It's super titanium and has a scratch resistant coating + a sapphire crystal.
This AW0100-51A is another option with a more simple design or maybe this more modern/minimal blue dial (has an integrated bracelet and you can't replace it for a strap if needed). There are others to choose from also.
For automatics there's the orient ray 2, bambino, maestro, pilot/flight, defender that might be within budget depending of where you live. Keep in mind that automatic watches are not as acurate as quartz and can gain or lose time /day as much as a quartz does in a month.
Citizen ny0086 is an ISO certified divers 200m watch that you can find within budget depending on your region.
Invicta 1953 (31290).
There are also plenty of homage type of watches both automatic and quartz (not many solars though) from brands like escapement time, seestern, addiesdive, SteelDive, Pagani Design, sugess, ets... that are good watches for the price. You can ask for recommendations on r/Chinesewatches. If you look on the choice section of aliexpress you might find some on sale but usually on r/Chinesewatches you gel a lot of direct links. Escape wheel watch reviews on youtube covers many of these in detail.