Best friend or brother in law?

Hey guys! So I love my brother in law. He’s just the best. He has his own issues, but who doesn’t? He’s 33 years old. My best friend, who is 20, gave him her number. He had no idea she was 20 until he took her on their first date...to a brewery and she couldn’t get in! She told me that she thought her age ruined it, blah blah blah. They’ve still been texting and hung out a few more times. He told me that he’d rather just be friends since she so young, but she keeps complaining that he hasn’t kissed her yet, etc. Should I tell her that he mentioned she was too young or wait for him to (which what’s going to happen is that he’s going to end up ghosting her or something, then she’s never want to hang out around him, even in group settings). I don’t want to start anything but I feel like it would ease her mind to know. So she’s not so invested and doesn’t worry so much about if something is “wrong with her”. Help!!