
Quoting
adjusterjack
I can't read your restraining order from here but I'm guessing that nobody is violating the "letter" of the order and it's certainly not illegal to use somebody else's address.
Your boyfriend's mother, unfortunately, is the one that has to deal with somebody using her address. The best advice I can give her is to pile up the mail, cross out the address, write "Doesn't live here" with a marking pen and dump the lot back at the post office.
When your ex-husband's family starts missing important mail and defaulting on unpaid bills, it won't be fun anymore and they will stop doing it.