I am not trying to scare goodwill, but don't ignore it even if it's just simply a flirt between your gf and him.

2 years ago I accidentally saw my ex-bf sending a messsage in French saying "I can't sleep 'coz I am thinking of you!". He told me it's just flirting but now it came true and they are going to marry soon.

Unless your gf is used to flirt people around, otherwise you have to think about why she is doing this.

Well either way if she seems to chat with this dude very often I would begin to suspect. I had a relationship broken off in this same way before. My ex-gf met some dude online and really chatted with him all day almost daily and very little with any of other friends. One day I just had to find some proof on what level is their relationship so I went behind her back and checked through her emails or conversations that she didn't know was backed up on her computer. Anyway after that was a whole mess of ups and downs for few more months since it was my first serious relationship.

So basically just watching her behavior will give you some clues since I believe body language is bit more honest than whatever they say to you.

THANK YOU ALL FOR YOUR RESPONSES :) y'all response so quick and just in time! KANBAN!! i think my problem is solved, she did said sorry to me and turned out that the guy is a WEIRD man. We both live in Sydney, if only the guy live in Sydney i would beat him til death, im a big strong guy :D [the guy live in Tokyo].
But what worries me the most is she'll visit Tokyo next year, so i think what's best is just dont think about it, if she do cheat, i'm just gonna get another girlfriend, PRETTIER, HOTTER AND LOVABLE [of course loves me more]

This is a good outlook on life. But remember that to err is human, to forgive, divine.