I'm not on FB but I noticed you said you planned to go see "them" this summer rather than "her," so this might possibly apply. (If so, he may have accepted your friend invitation years ago to be polite or whatever and now is getting around to culling his FB list). This, coupled with you being upset that he unfriended you on FB, made me think of this:
If it applies, I'll just say that people have different ideas of boundaries. For ex. I don't like it when I make a friend and they assume they are friends with my husband, too. They are not and to me, it's an annoying, uninvited overreach.
Sometimes they do have another agenda of wanting male attention from him to one extent or another and other times, they don't but it's still irritating and will make me decide to not further pursue the friendship. It's happened the other way too and my husband feels the same way I do. In other words, my friend would be coming to see "me," not "us." His friend would be his friend, not "our" friend.
Now that my kids are grown, it's come up there too, once or twice, where a friend of mine or my husband's tries to make an independent bond with one of our kids. It just seems pushy and annoying to us, and our kids don't really want to hang out with people their parents' ages either.
No idea if it applies or not but just a thought about if you're inadvertently crossing the boundary that your friend actually offered you there.