well, I'd like to say no, but sadly I'm only human!!!
It did happen on many occasions that after lending a helping hand to a friend, he/she did turn around and bit it. However, it does say more about them than me.
I try to be a good friend to others by putting their best interests before my own when they ask me for advice. If you are being honest with yourself is not actually that easy to do.
I am far from perfect but I try to abide by those rules...when I can anyway. But what I never did and refuse to do is stand between a friend or any person for that matter, and a good opportunity. I refuse to ruin things for people...
I have been betrayed by friends many times and I find it hard to trust people nowadays...I guess that true frienship is rarer than diamonds.