If she is underage he will certainly be charged with "having sex with a minor" (in Canada), not sure about a "john" being charged if she is being "forced" to work.
If she is being forced to work then she could not possibly consent to sex. When he's brought up on sexual assault charges what's his defense going to be? "Her pimp told me she consented", that is going to elicit a rather large laugh from the judge--who seriously believes anything a pimp says??!
If there's an easier charge to go after, like a bawdy house charge, the prosecution will likely take that instead just to get a quicker/cheaper conviction, but really, fucking someone you should have known did not really consent to sex is some serious hot water.