Most Helpful Guy
If that girl / woman is obsessed with him it's a bad call to involve yourself with her.
I've had cases of women stalking me (guess it's ordained) even if we meet as an friends with benefits or even as a simple conversation or worse - being kind to her for any reason and have faced suspicions & nightmares from my partner in the past for no fault of mine. Even going to the cops and complaining, getting a restraining order wouldn't satiate my partner cause she was insecure herself from within.
If he's being earnest about letting you know about her calls and stuff, I'd say believe him.
If you do want to find out if there's any truth in your suspicions, the safest way is to get one of your smart, strong and bold who has good worldly knowledge woman friends to be friends with her & get some facts out of her w/o asking for it cause she'll spell it out herself (believe what she can prove nothing else cause she'll probably state things in her mind that haven't occurred as facts)