V, as a mom of both a teen son and a pre-teen daughter, I have one thing to say: YOU DID THE RIGHT THING. The girl's mom probably had some clue things were gettin' hot and heavy between her daughter and your son, hence them having him over to meet him, but even if she didn't she needs to know just HOW hot and heavy things were gettin'. I know if it were my daughter involved, I would want to be told if her boyfriend's mom knew something I possibly didn't. Does that mean that it would be a comfortable conversation for me? No. Does that mean I might go quiet as I try to figure out WTH to do with the information? Probably. Don't sweat about it. Just try to make sure all involved know that the lines of communication are open on your end and that you're willing to discuss this with any/everyone involved.
HTH