S.H.
you grew apart & now you're trying to "beat a dead horse". NO single person is at fault....life happened & you all made choices.
In your post, you said, "WE met many other people & friends & stopped getting together as much". The originating issue was when you started spending time with other people. & in all fairness, perhaps your "WE" meant Joe/Sara, too. :)
Fast forward to Joe getting a replacement for his truck...good for him. It was an adult choice to purchase something more effective for his needs. What would have happened if your DH's car broke down? Joe made a great choice.....+ he knew that you guys were pulling away with new friends.
Yes, it would have been nice if he or Sara had offered to carpool with Joe's car..... but then again, you were pulling away. It concerns me that you were "upset" & felt somewhat "used". That's not what I would have been feeling! It's almost like you were looking for reasons to find fault with them! It amazes me that you could even "wonder" as to what had happened to change things.....again you had new friends! At any point, you could have included Joe/Sara with your new friends! That's what happened....you left them out!
You know, the further I break this down...the more it sounds like H.S.!! I quit - I'm going back to my original paragraph.....:)