Boyfriends best friend always hitting on me...Help!

Okay so I'll give a little background story to fill you all in on the situation. About 2.5 years ago I met my current boyfriend through our group of friends. They are now 30 yrs old and I'm 19 (so there's an age gap but they are like my older brothers is all!) Well I was friends with his friends long before we started talking. Anyway, his one friend in particular is his closest friend. They have been since high school and even lived together for a bit. The problem is, his friend has always tried to get in my pants. We can hang out by ourselves and be fine but he usually brings it up at least once. He will sometimes snapchat me with "anal?" Or like grab my butt when I'm leaving. My boyfriend is aware of this and gets kind of nervous but trusts me. And I would never cheat on him. Thing is, he had a girlfriend in high school that cheated on him with this 'friend' so that's why my bf worries. I guess I'm wondering what I should do? They are good friends and I don't wanna be the one to break that up. It doesn't really bug me a whole lot cause he's not getting any of this ass lol plus besides the fact that he does this, he has always treated me like a little sister. (Been there when my car broke down, followed me home in a snow storm etc) and he cares amd watches out for me all around. Should I not hang out with him as much? Should I just let it go and ignore the sexual comments?

Sorry that was so long! Haha