Mermaids are the apex predator & rule the seas - none of the critters are friends, just captive slaves that try their best not to piss her off.

And that's why a little murmaider is ok.
\m/

Maybe the ones she uses as bra are her very best friends?

A bra can be a friend.

Clearly her bra are her closest friends.

Well starfish have their mouth in the middle of their body, so...

Simples, when she out grows her B-shells, she wears her C-shells.

She is like a hermit crab, using only the discarded shells as her cycle progresses.

How do we?

I’m assuming the same way we decide which animals are okay to eat or not - arbitrarily and by culture. For example, maybe some Pacific tribes of mermaids might be fine using starfish, but Atlantis area that would be considered barbaric, perhaps even with a touch of propaganda saying they use starfish for sexual reasons as well. I love where this is going, I’m gonna go write a controversial AO3 fiction about a Pacific mermaid fighting against Atlantic racism, but secretly in a degrading race-play relationship.

¿Por que no los dos?

I would imagine that the ones cupping her titties are "more than just friends".

Alive: friends. Dead: clothes.

Well, obviously her bosom buddies are her friends! They just clam and do their job.

Edit: when you think about it though, do you really want a couple of clams in charge of guarding your titties?

The person who wrote this is in the very small minority of people (of all genders) who don't think boobs are awesome.
And even then, how is being her bra not being a friend? Bad bras that cause all sorts of issues and discomfort is such a common thing that any bra that doesn't should automatically be seen as a friend.

She only wears the dead ones, dyed a bright color.

Bra ones got game

I'm not sure there's a way to do that without being shellfish.