Rida,
if he is only looking then I have to say this...
ALL men look and if any man tells you that he doesn't he is a liar! Some men will look at other women right in front of their significant others thus making it obvious (these men are total losers) and some men will look subtly because they do not want to hurt the feelings of their significant others..then there are those that are trying hard not to look (perhaps for religious reasons) but they still have the urge to....and they glance every now and then.
Now....if your man is paying too much attention to another girl...meaning it is gone beyond just "eye candy"...then you have a problem!
Firstly, whatever you do... DO NOT show him that you are jealous. Be confident in yourself (even if you are not...fake it) Let him know (by the things you say and the things you do) that you are the best thing for him and no other woman can treat him better. Do not show him that you are insecure, self conscious and/or jealous. some men take advantage of this.
secondly, if he is disrespecting you by taking interest in another woman right in front of you.....ignore it! Address this issue at some other time....not right then and there.
Hate to say this but....some men are just losers and they want to make their women jealous....dont fall for it!
one more thing....of course addressing this issue is the key factor but you have to do so strategically. Like I said above...be confident in yourself (no crying or getting jealous)...let him know how you feel. If he cares for you he will put an end to it...if he doesnt then it's his loss not yours.
Good advice. Only thing I will like to add is that - if talking directly fails, try bringing elder's into this to sort things out (sometime you need a mediator).
I am just throwing an idea - don't know if its suitable for you? You can try talking directly to the girl to cut it cold and move away. I am sure, guy won't like it - but if the girl have some decensy....