Asked to be "Second shooter" sounds different to me than asked to help out with everything on the day as a general assistant.
I agree with the others, there should be no reason why the 'pro' wouldn't say yes to you using your photo's in your portfolio, and if they say no and you feel strongly about it you can politely decline their offer. Asking still sounds very different to telling if it's their gig.
Er, in reference to your post above, it implies that you gave away your only copies of images you had taken? Surely not!
If you had no agreement at all you are entitled to do whatever you want with your images. If he came up to you and asked you to take stills for his video and you received something in return for that with no written agreement it's shakey ground for all concerned. I'd write more but rights are covered better in other threads and elsewhere I'm sure.