I believe he means that he doesn't think you should cast the spell for your friend.
In fact doing so could ruin a friendship. Say the spell doesn't work for some reason, or it works in a way that your friend is unhappy with. He could easily blame you for what happened or even decide that you worked "bad" magic on him. Next thing you know your friend isn't your friend any more.
If he can do the spell himself then encourage him to go ahead and do it himself. That way if things go wrong he can only place blame on his own head.