I don't want my girlfriend to go on a camping trip with her male friend alone
That sounds suspicious. Why do you not trust her or something?
I'm afraid the guy may try something funny with my girlfriend
It is reasonable that you don't want your girlfriend to go on a trip alone with another guy. You should talk to her rationally about your concerns, and offer alternatives, such as asking to to go on the trip yourself.
Nah, I'm going to tell my friend to go spy on them. I never said I was going to be rational.
If you have someone spy on her she might think it really crazy. Please just talk with her, there really is no good reason for her to go on a trip with a guy unless they are coworkers on a business trip.