Hey Monica...from past experience on this same thing....the *other* mom will side with her son/daughter...I know this. It is natural for them to feel this way. No matter how much info they get...it only stays bitter with them for a very short period of time. The letter to his Mom sounds nice, how you put it, but again....it probably won't make a difference...maybe piss him off if anything else.
See, the thing is is that MIL is not going to spank him and tell him to grow up.....that is the problem...that is what the idea behind sending her the letter is driven by. Truthfully....whether you want to admit it or not....no matter how much this hurts you, she can't do a damn thing to straighten him up.
It hurts....seems like there is nothing you can do....nobody to wake you up from this crap. It pisses me off just to have to type this out to you but like they say, the truth hurts and this hurts...all of it.
Blast of reality.......YOU are the bigger person here and this action that he is goping through is his problem....ultimately drawing you in too......
Time to be strong Monica...slap in his face may just have to be papers.....