Well, I'm with you mom because I don't understand why you have a problem with it either. Sorry, but your mom is completely in the right here. She can hire whoever she wants to do work for her. It would be different I think, if this guy had been abusive or a total louse to you but you don't say that's the case.
You say you are happily married, have moved on and have no feelings either way towards him right? If all that is true then this shouldn't even be a blip on your radar screen. How on earth is it disrespectful towards you? Obviously this guy wasn't some monster while you dated him or you would have said, you have been with your husband long enough to have 2 kids. So I just don't get what the big deal possibly could be.
There are far bigger things to get upset about in this life and this ain't one of them.
I applaude your mom for thinking of helping out a family friend who could use the work and money in his pocket. The world is tough enough without being vindictive over nothing.
If my dad hired one of my ex's to do work around the house I wouldn't even think twice about it unless it was the one guy who was a total jerk. Again, you aren't telling us that this guy is/was that way so no reason to be upset. I'm friendly with several of my ex's. I wouldn't run the other direction or be mad if I saw them somewhere but I also don't go out of my way to see them either. I think of them just like you said-"don't really feel ill-will or any positive feelings one way or the other".