Is it inappropriate for a 26 year old male to be friends with a 17 year old coworker?

For context: they are coworkers. She’s really sweet and I know the friendship is not romantic in any type of way shape or form. It’s my boyfriend/father of my children (one who is 3 months old) and he is currently training her to take over the assistant manager position at their work.

I’ve always been friends with people of all ages so that’s not the issue for me.

The issue I have is that she will pick him up from our home after she’s “had a stressful day at their job” so that he can go to the store and buy her and her friends liquor. He bought another female underage coworker liquor before her prom before too) He gives her relationship advice. He’s told me her and some of the other highschool girls at his job have talked about how they want to have threesomes before. Last night he was on the phone with her for 45 minutes helping her close their store since I guess something happened and she didn’t know what to do.

Next thing I know he comes in the room and tells me that he’ll be right back because she was picking him up so that he could go buy her cigarettes and vent about her night at work.

To me, this is completely inappropriate especially because he’s her manager (and she’s underage obviously so it’s illegal and we have KIDS so why would he even risk it) and I believe he is completely crossing the line.

He doesn’t seem to grasp why I feel this way. Am I just the odd one out here? Like I said, I have no issue with being friends with people of different ages. But I do think there need to be boundaries and that it is completely unacceptable and it’s lowkey starting to really weird me out. I feel like he’s just trying to be the “cool manager” but at what cost? Am I just a lame AH?