So let me get this straight.....he's been in rehab something like 2X, this year. And once again, you are OK with having him in your home, only this time it's going to be different, eh.
This whole GF thing sounds a tad convoluted. Exactly where does it become her responsibility to tell you what's going on with your son?
Your son broke your house rules and somehow, the GF is no longer welcome in your home ( unless she attends Alanon or something) because she did not rat him out to his mom.
Hey, it's your house and your rules and all that. Just having a tough time wrappoing my head around how the one who broke the rules is back in the saddle again while his GF is no longer welcome. Regardless, they both sound too old to be hanging out at mom's house.