They are giving out their phone number to people they met on roblox.
The child is institutionalised, their parent cannot control their behavior meaningfully and the adults at the institution are pretty negligent and she doesn’t trust them.
I have messaged like my whole family and some friends for advice but nobody is getting back to me.
Potential online abuse could be an outcome and that would be terrible, but having a serious talk with her about the dangers next time you see her could go a long way.
On the other hand, losing her only connection to the outside world in a negligent institution would definitely be terrible and could lead her down a very dark and hopeless path. Avoid that at all cost. The parents would probably just tell the staff and they would definitely just take her Internet access away and be done with it. Best case they talk with her when and if they can make time for that and that’s something you can just do next time.