what will you do if somebody told you that your GF/BF is having an affair?

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The girl have a fiance (from USA) and she will be having her embassy interview soon. This girl happens to have 2 account in a social networking site and somebody who knows the girl saw a picture of the girl kissing another guy (a real real real kiss). This is posted in at 1 of the sites. It seems that the 1 site is for the other guy while the other 1 site is for the girls fiance. This somebody (the one who saw the picture) save the picture and told the fiance about it. But the girl told her fiance that its a friend of her friend that is trying to broke them apart. But here is the scenario: The profile of the girl where the picture was supposedly seen is in private profile. So if "I am the friend who is trying to broke them apart" I wont make that profile private. I will let everybody to see the picture. But they said "fiance" believed the girls story. If it was you. will you still trust her 100%? Wont this kind of issue give a relationship a doubt? I myself can believe the girls story because I myself witnessed something like this.... any opinion????