Going to strip clubs will lower his value on women and give his a unrealistic idea of women and sexual activities. It is a place to go and lust after various naked women. ( plus ever wonder why the lights are low at these places, if the lights were bright at many it would scare the men away)
Not all of course.
Now will he sneak out one night after he is 18 ( and honestly the lower class ones will pass you at 18 for a few bucks at the door) and go once, I guess it is a stupid things kids ( young people) do once. Some never go, others never go back but have the one time story to lie about the rest of their life.
Guys are stupid in that way.
But it is cheating in many ways, the same as in web porn or regular porn is cheating, it is taking away the emotional being of the person you care for.
It is not the one time but the aditction of going I would worry about.
And honestly at least he is being honest about it, most guys would not be close enough to their girlfiend to tell them they were planning on going but just sneak out and never tell anyone.
My opinion, no reason to go esp if you have someone speical saying not to.