My question is can I eat salad or fruit and the house of my friend (she is not Jewish) or at the salad bar in a restaurant that is not Kosher ?

You may eat in a friend's house, if there are no problems of bishul (non jewish cooking). Fruit & vegetables - if cut must be cut with a kosher knife; and must be checked if there is no problem of trumos & ma'asros (if they are from The Land of Israel), if there is a problem don't eat them all, leave a bit on the plate. It is inappropriate to enter a non kosher restaurant at all, for 2 reasons: 1. You never know what they put in their food. 2. Other people seeing you might be led to believe the place is actually kosher. Rabbi Ro'i Margalit