I'm really quite surprised by the number of people condemning this guy for checking his GF's yahoo.
I know so many people that have sneakily checked on their lovers, and ever single one of my girlfriends, has, at some point, snooped through my stuff, whether facebook, emails or phone. I can think of two females - just off the top of my head - who found out their partners had cheated by doing this.
I find it an entirely understandable behaviour, and not one I would condemn anyone for. I don't think it's highly admirable, admitedly, but I'd put it on a par with the negative emotions of being jealous, nervous or worried, for example.
OP - do you have a transcript of what was said between your GF and this other person? Sharing it might shed more light on the situation and help you get better advice.