His gf is jealous of/ dislikes me

C
I have a guy friend that I've known for a long time (about 22 years/ since age 5). Our relationship is platonic, not that close, we hang out and go for drinks every few months with other people, or just the two of us if those others aren't around. I have never had feelings for him and I guess would consider him like a bro. He told me once when we were 15 that he liked me and I immediately shut him down and never entertained or encouraged any kind of romance, but carried on with the friendship. He has a girlfriend who in the last couple of years has become distrustful of our friendship and I just learned that she is worried that he has feelings for me. She doesn't like me and doesn't approve of us hanging out. (I dislike her for separate reasons but would always make an effort with her when she is around) He continues to hang with me, and the others, bc we are his main friends in our town, his friends from school, childhood etc... 
I would like to try to help him to help his girlfriend to not be threatened by our relationship, because... there will never be anything between us. I am not at all interested and he drives me insane with his stupid behaviour sometimes, etc etc. What can he do to help her stop hating me and our friendship?
I should say that I've been the jealous gf before and know how horrible it feels, but I've never had a girl like me on the receiving end of the jealousy, I.e. someone that she does NOT need to worry about.