Actually he wants me to loan her bitcoin/money (to help her buy a flat) but explaining to him that if I lend her bitcoin she would probably never be able to pay me back is pointless.
Never lend to relatives if you can possibly avoid it (especially if your culture isn't set up that way). Likelihood is that you won't get paid back and it will destroy your relationship.
Your Dad should lend the money himself if he thinks it's a good deal.