Ex's best friend wants to have dinner?

Hi guys, so I’m wondering if anyone has seen this happen before…
So my ex only really has one friend. They’ve know each other for prolly 15 years and lived together for about 12-13 of those years. My ex is extremely close to him still even though his friend is sorta getting on with his own life. My ex is close enough that now that we broke up he just rented an apartment he can’t afford I’m thinking only because it’s on the same street at his friend… And since he doesn’t have me anymore, the loneliness is getting to him more than he expected so he’s justifying the cost to be close to his friend. But that’s not the point. So his friend and I aren’t particularly close. We were technically roommates for a while but by the time I started dating my ex he’d already moved in almost permanently with his fiance. We only ever saw each other or went out together if my ex was there too. As problems started to arise in our relationship, this friend always told my ex that (given his age) he needed to realize that no one is perfect, this girl REALLY cares about you, and basically to suck it up. My ex listened to a point, but since he was also convinced his friend was with his fiance because he was “settling” I’m not sure he ever really believed him. (The friends fiance is basically a polar opposite to the two of them but it’s obvious that he will always be there for him forever). So just to be nice the other day, knowing that his friends fiance LOVES star trek, when I went to an antique shop and saw a limited edition star trek monopoly set, I contacted the friend and asked if I should get it. He said yes. So I went back this Wednesday and picked it up and told the friend to just let me know when he wanted it (I expected him to just want me to drop it off or even to give it to my ex to give to him cause I know MY best friends don’t want to see my ex anymore). Instead, he asked me to come to dinner! My ex’s closet personal friend whom I’ve never really hung out with alone, who I know was very pro-us, wants me to come to dinner this Sunday! I can’t imagine why, he’s never wanted to have me over before and he may be one of the only people my ex knows who is aware we’ve be broken up for near two months. Recently my ex and I have been talking everyday, I don’t even have to instigate conversation. Not about the relationship, but about almost everything else. I’m not sure what to expect this Sunday. I don’t know if my ex even knows I’m going. Or maybe he does and he’s having his friend tell me something. Or maybe his friend is just so sick of my ex bugging him all the time now he wants to know where I stand on getting back together with him? ( Especially since he always liked us as a couple anyway). Has anyone else been through this?
Thanks,
-S