Wouldn't she be more upset if you gave him the same kind of 'loving support' she was giving him in your driveway? I bet he would too!
(Reminds me of when Wiz was about 7 and I used to tell him that if he didn't stop doing something, I would kiss him. At the time he was in the 'I'm too grown up for hugs & kisses from Mom" stage. I found it worked very well, lol. But it was NOT what the girlfriend was giving your son, of course.)
This 'advice' from the girlfriend is ridiculous.
Even if your son gets little action this time from the charges, you still need to do this and keep doing it each an every time he steals from you. The amount and frequency will only go up in the future, esp now that he has a girlfriend & a baby & a baby momma. All that stress will mean he 'needs' more munchies in order to cope, and since it is 'only' for some snacks, it isn't a big deal to use your credit cards. In his mind, at least. You have to be consistent & follow through, even if you don't like what the court does the first few times. I am sorry he refuses to act responsibly.