Why would he pay a fine to an offence he wasn't guilty of? You do know that if you pay a fine, it's the same thing as pleading guilty right? The whole story of him just picking up a girl on the street and it happened to be his friend, and instead of fighting it, he pays a fine (pleads guilty), and to top it off, he's also an illegal. Oh boy, you've got a long road ahead for his PR. I really hope you're not just trying to find excuses for him if he really was picking up a hooker, because if he really did, then you have to re-evaluate if you really want to be with this guy. Also, some men take advantage of women who turn a blind eye to bad behaviour, if he really was a bad guy, taking advantage of you overlooking the prostitution charge is just the tip of the iceberg, he might try to take advantage again by getting canada PR. You should definitely consult a lawyer and just think about this whole thing carefully.
Good luck to you