Don’t try anything. Just let her go. But make sure she is aware you are there should she need anything (other then financial help – like you won’t just give her $$$). Don’t make her feel like she has to sneak around to see him or belittle anything about him.
Just tell her you accept the fact she is an adult. You don’t like him (not for the real reason) because he doesn’t make her happy and isn’t there for her. Remind her a relationship is 50/50 and subtly ask her where his 50% is. Just be able to be there when she falls.
(but don’t word it like that) Remind her you are there for anything she needs. “armstrca”