Know When to Throw in the Towel

I am a very determined person.  Some people call it being stubborn.  I like proving people wrong when they say I can't accomplish a goal that I know is well within my capabilities.  With that being said, I have been working on a situation for a friend.  The goal can be easily accomplished by my standards - it's just a matter of the friend agreeing to the situation.  Said friend refuses to do so.  Said friend wants to bend the rules of the agreement.  I have tried reluctantly to explain this is an all or nothing situation - the rules cannot be changed to achieve the desired goal.  At this point, after months of trying to help, I am ready to throw in the towel.  You can't help someone who does not want to be helped.