I would definitely make sure you clear up the vague answer he gave you. I used to do wedding photography here and there and as a result became known as the photographer to family and friends. I always had to make it clear that I am either a guest at the wedding or a photographer. It was up to them to choose which role they wanted me to play. I did not mind either way, but I wanted to be certain of what I was supposed to do.
If I go to a wedding as a guest I might take a few photos out of habit and whatnot. But, it's near impossible to enjoy the ceremony and know what is going on if you are snapping photos every few seconds.
Honestly, if I were you, I would hire a photographer or use the resort one. If you invited this person as a friend it is kind of crappy to then make them 'work' while there. Not saying that is what you are doing, just something to keep in mind. I know I would be pretty offended if I was invited as a guest and then had the expectation of providing wedding photos added on after the invite. If everything is made clear ahead of time feeling will not be hurt and you will get proper wedding photos one way or another.