Use your words.
Talk to your son, don't diminish your position by letting him know you searched his girlfriends pockets, or that you don't like her.
Speak to him when she's gone - mention that you could smell the weed, be firm you know that is what it was, you're not thick...
Don't slam any doors on him, metaphorically speaking - if you lose your shit, you stop him talking to you and being honest with you.
Despite being an occasional (and I do mean occasional, for anyone following the edibles via online shop thread from a while back I still have most of the bottle I bought then!) cannabis user, I think you need to tread a fine line between 'lose your shit' and 'cool Mom' here.
Some people can absolutely use weed occasionally, with no ill effects, no addiction issues, no psychological problems or depression etc etc. I know, I am one.
Some people will develop all sorts of problems and it will do them no good at all - there is no real way to tell which is which until they start using it... I got lucky there (I didn't with alcohol so I never drink now!)
Unfortunately by the time you do develop problems, those problems make it very hard to stop using weed, and hard to ask for help too.
If you make sure he feels he can talk to you, that he understands the risks, and point out any relevant factors for example, pre-existing mental health problems or physical health problems, that would increase those risks, then you stand a much better chance of him making the right choices, and coming to you if he makes a mistake.
Ultimately you can't stop him trying it and using it, all you can do is be there and help him navigate through it.