My step son was living way beyond his means and had married a woman who refused to work.
He asked if he could barrow 10k (that we had set aside for new windows) as he wanted to start his own business. I felt I had no choice but to agree as he is my DH's son. He only wanted it for two years and we weren't ready to buy the windows yet anyway as we needed to save a further £3,800.
After 4.5 years of no money back, no starting his own business but him immediately going on holiday and buying a BMW (which he then chopped in for a merc), DH asked for it back but nicely.
We received a letter (all in capital letters) saying no, he had no intention of paying it back, doing so would bring him and his wife 'genuine hardship' (in their new off the plans three bed detached) and we were dumped from their lives.
I called him, tried to reason with him and when that was futile, told him I would take him to court with his letter to us as 'exhibit A', and the money appeared in my account within the week.
On their exit from our lives, he managed to persuade his sister (Dh's daughter) and DH's sister to dump us all too so we had to quickly change our wills as we had left everything to all the people who had cut us out.
That was on expensive loan in every sense of the word.
We've heard nothing in the last 9 years. Just the occasional meme on Facebook about toxic fathers and shizz like that.
So glad I found out what they were like though. That is an upside.