we can help we won’t charge anywhere near that.
Often we’d said add it to the invoice
You thought you were saying jokily. They didn't, but from the first I can't really see how you thought it would be free.
You assumed because they were friends that they couldn't possible charge you. That's you being cheeky.
It's irrelevant that he did a bad job, in your opinion, although I'm also wondering if you've added this in to try and make yourself look better, because it's odd that a trained joiner does such a bad job but you can make it better after he's gone - so clearly not that skilled to make it better.
He has made a dreadful job, so as they’ve come back I’ve redone them myself and not said anything.
If you could do it yourself, then I'd also think you were bring pretty cheeky letting him turn up, over a period of four months, and let him get on with it. If you'd been doing it together as two couples over a couple of weekends and having lunch and a nice social time then that might be different.
Before he started you should have asked him for a quote, insisted that you were using his skills so he must charge, and let him make a decision on mates' rates for his wife's friend.