I don't know if it's true or not, but I'm going to tell my friend that his girlfriend didn't randomly spend $50 at CVS
Why are you accusing her if you aren't certain what happened?
It's possible she did or I'm just being paranoid.
You shouldn't concern yourself with what your friend's girlfriend does. Interferring in there relationship will cause problems between the three of you.
He's just a friend.
This is a tough spot to be in because you want to help your friend, but you also shouldn't get involved in their relationship. I think it would be best not to mention anything, money may not be an issue for the two and what you saw was normal behavior.