No 
On a serious note, depends upon why the woman is looking. Sometimes you know that divorce is inevitable so if you start screening out some potential guys then it might be ok but if you start dating him then I think that will be cheating. Not only that, you will put yourself in a stupid situation. One might be divorcing her hubby cause he physically abuses her but now he is gonna go out and tell people that my wife left me because she was seeing this guy while we were still married.