Ug, your story has brought back memories. There is always some kid who doesn't "trade" well. They make the trade and then go back on it. I know my boys each had their share of that experience.
I have had a couple rules when it comes to the kids and other kids and belongings...
1. If you loan or trade or leave something with someone else...expect that it is gone. Don't be angry, but just accept that it was the price of playing.
In other words, don't play with something you are not willing to lose.
2. If you do play with kids who think you break their things, take their things or the reverse, they break yours or take yours... you don't play with them any more. In other words, if it is a person you don't see eye to eye with. Because in someones eyes they are in the right while the other one feels they are.
I know that is easy to say.. especially for me when I wasn't a part of the PTA. I just steered clear of the parents of these boys as much as I could.
I am sorry that this is someone who is suppose to be a friend of yours too.
I am also sorry that you have to be the one to see where your child has been wrong while the other parent feels their child has done no wrong. (I have been there too) That is so frustrating and disheartening. For me it always made me feel bad, like why wasn't I the parent who just thought my kid was an angel.

Hang in there... as much as I hate to say it... you both are going to be better off if you stay away from this family.