Another guy likes my girlfriend! HELP?

Alright me and my girlfriend have been together for about two years, and we are very happy (most of the time :) ) and there's this guy who is a "close family friend" that she went out to the bar with a few times and he said he liked her and wanted to be together, she told him that she was happy with me and denied him. she recently told me this and normally i would just be like well you shouldn't probably see him anymore, but he is deployed in Kuwait and will not be back for 9 months so i didn't think it was a big deal, Yesterday during a VERY EXPENSIVE dinner i bought her he called. from some military phone whatever, she excused herself and talked to him for 10 minutes, i confronted her about it and she explained it as hes lonely oversees and has no one else to talk to. Is this a big deal? and what about in 9 months when he comes back should i be totally ok with them hanging out together even though he likes her? thanks in advance.