I think my best friend still has a crush on my girlfriend. What do I do?

Trust your girlfriend to handle her own business.

Like, seriously, if you’re both adults (and even if you’re dating as not-quite-adult adolescents), she can shut him down all by herself with something like the following.

Partner’s best friend, I understand you’re attracted to me. However, nothing’s going to happen between us. Please respect the relationship between me and my partner. Thanks for being a considerate friend.

That lays things out clearly and unambiguously for everyone. Your friend might need some time alone to process this and get over their hurt feelings. Let them. Don’t push to do group things together as a trio since it’s not just friendship for your pal.

Eventually, if your friend is a decent sort of person, they’ll get the message and accept the situation.