My friend says they're polyamorous, but is not doing it right

I’m gonna sound hateful or like an anti but please hear me out. My friend has vented to me before that they have issues with commitment and are scared of being in a relationship because it means they’re "trapped", now they’re apparently poly but I feel like they actually aren’t and are just making things worse for themself by dating a bunch of people and keeping it secret from all of them (so each partner thinks they’re the only one) What do I do

You could do nothing, because this isn't your circus or your monkeys. Your friend being wrong and making bad choices is not your problem or responsibility. Maybe they're polyamorous, maybe they're not; it's not your call to make, and being skeptical of someone else's self-stated identity is rarely a good look. 

The bigger issue is their behavior, which is decidedly not polyamorous, but is cheating. Seeing multiple partners who all think they're the only one is immoral and cruel. But, again, there is nothing you're obligated to "do" about this - you did not make this choice and you cannot do anything to change or mitigate it.

You could tell your friend that what they are doing is wrong, and try to convince them to stop. You could take a step back from this friendship, not because you disagree with what they say about themselves, but because their behavior makes you question whether you want to be friends with someone of that character. 

You could get more involved and tell their partners what is going on. Some people feel a moral imperative to tell victims of cheating about the situation. Some people feel strongly that it is not their place or business. Some people just have to decide on a case-by-case basis how much they want to get involved.

You have to figure out whether you want to take a huge step back from this whole mess, or get more deeply entwined by reaching out to the people your friend is cheating on. That would likely torpedo your friendship and establish you as an active participant, emotional baggage handler, mediator, etc. for all the people whose hurt you'll be bringing to the surface. That's your call.