My advice is to learn a lot as much as you can about polyamory, and how to go about it in a healthy succesful way. Until you’re equiped to understand how different the relationship dynamics can be it will be hard to give useful advice or support, or know when things are not in a good place.

Polyamory can be a perfectly reasonable and healthy, if untraditional way of doing relationships, but it can also very easily be unhealthy, especially if the relationship is polyamorous for the wrong reason.

Ultimately what would be most important to me is that my kid is healthy and emotionally safe, and until you know enough to evaluate those things, it will be very hard to know whether your kid is in a good spot and navigating a style of relationship you’re unfamiliar with, or being taken advantage of while navigating something that is also likely very new for them, or somewhere in between as everyone involved navigates uncharted territories as young adults, making mistakes along the way (which to be fair, is developmentally normal.)

I very much agree with the other comment about twice the risk. I had a very painful experience with attempting to be poly when it was a poor fit for me and my then-partner, but I have friends who are poly who are profoundly happy to have found the type of relationship that is fulfilling and feels right for them 🤷‍♂️. Its not for everyone, and there are definitely ways of going about it that are likely to end in heartache, but for many people it’s a revelation that there are no rules for love, and that they’re free to assemble whatever type of relationship they feel is right for them.

I wish you the best friend, parenting a child who is neavigating things you are fundamentally unprepared to help with because they’re alien to you is more than a little bit scary. The best you can do is learn a lot, try to understand where they are, and try to support them in building a healthy life with healthy relationships, even if it doesn’t make a ton of sense to you.