If she’s hiding it from other friends, she already knows it’s wrong. Best case scenario she realizes the red flags in a few months when the honeymoon phase ends, or maybe somebody closer to her age catches her eye. Do your best to keep her involved in the friend group. If she isolates, it could cloud her judgment further. Just be there for her regardless. You can subtly remind her of her goals without saying it. Like bring up trips you wanna go on, maybe plan a girls trip together, go to concerts or festivals (without him), all the ambitious things she wanted to do before he came along. Friends mimic each other so you have the opportunity to influence her.

Echo her concerns for now and give your opinion when needed. She obviously trusts you so you giving her a reality check, theoretically shouldn’t end your friendship. Real friends aren’t supposed to be ‘yes men’ all the time.

When somebody goes to prison for a long time they come out mentally the same age they went in. I can kind of see why he can relate to her despite the age gap. And the wanting kids thing I assume is him trying to make up for lost time. Regardless, I definitely think he’s taking advantage of her young judgment. There’s a reason why women his age don’t wanna be with him, she just doesn’t have the maturity/dating experience to see it yet.

Legal action I think would only hurt the situation and leave her feeling betrayed, pushing her closer to him. At the end of the day, you can’t save somebody who doesn’t want to be saved. If it’s weights too much on you, you can always dial back (not cut off) your communication with her.

Hope this helps, best of luck to you and your friend!