Problematic result of working for friend - advice please

Hi everyone - new to the forum - perhaps someone would like to let me know what they think of the following story:

I currently make a living in part by writing marketing copy on a freelance basis. I am relatively young and my daily rate is on the lower end of the scale for this kind of work.

A friend of mine recently got a job with a large, publicly funded corporation. Soon after, he offered me a piece of freelance work, which I was partly qualified for (writing) and which involved a task that I was inexperienced at (although we both evidently thought I could probably manage it).

I worked very hard on the job, and my friend initially told me that both he and his immediate boss were happy with it. Subsequently, he told me that he'd gotten in trouble because the second part of the job was not up to scratch, and that this has damaged his prospects of promotion, something that he is working very hard for. He sent me a very rude email about this, which he later apologised for.

Now, he says he needs to hire someone else (more expensive than me) to redo the latter part of the work, and is on a tight budget. In order not to look bad to his boss, he wants to pay me for fewer days than I actually did.

On the one hand: I want to help my friend do well with his career, and I appreciate him giving me the work in the first place.

On the other hand: He should have known what was expected by his employer - it wasn't my fault that what he submitted to his boss was not up to scratch. I don't think friendship should necessarily extend to working for free. And I don't think it will really be the end of his career or anything if he pays me the full amount.

What do you think?