I am confused. Because they traded cars in the past you are assuming that was permanent and neither car can break down or need to be swapped back, etc.? They're cars! If he is using "this" one, he should be in it? I don't know where you saw her and if she was in driver or passenger side but she could have her own new boyfriend and he was getting something.

If he gets to see his kids now because the courts say so, it is quite probable he can see/be in contact with the wife as she has the kids and going through a third party could be costly or, the "supervision" could be through the wife being there too. If you do not know all that went down, have not been in contact with him to know completely what is going on with him, I don't know that I would jump to the conclusion that he is back with his wife, is "cheating" on you?