Track when the business fails. Lightly share your concerns, but don’t push the issue. Show them that you are sympathetic to them and that you sincerely care about them. It will be hard when they have to try to make up for their poor choice, and having a friend will be Important.
It is unlikely anything you say to dissuade them now will help. And it is very likely it will make you seem like an unsupportive jerk.
This is their mistake to make. Be kind and thoughtful. When you make mistakes, it is how you would want to be treated.