I suppose it depends upon how much of a pledge it was. For example, if an honorable man makes a pledge (vow) to a woman in marriage, it really doesn't matter what she does in the future. An honorable man is still bound to keep that pledge.
On the other hand, if you happen to tell someone you'll give them $1,000.00 to pay off a debt, and the person immediately spends $1,000.00 on something else and doubles his debt, I don't think you are necessarily bound to give that first $1,000.00 to him. It really depends on the way the "pledge" was made. For example, if he came to you asking for help to pay off a debt and you tell him you will help, then I would not hold myself to be bound since it now appears he really just wanted the $1,000.00 to pay for the new item he couldn't afford and it was really just a con-job.
If there was a "friendship" of sorts, that friendship is likely to be over, but such a person probably isn't really a friend anyway.