What would you do if somebody refuses to pay there half of a debt?

I would ask why they’re not going to pay, whether they’re unwilling, or just unable.

If they’re just unable, try to find a way to work things out, whether it’s me just telling them they don’t have to pay and then covering it myself, or me giving them extra time.

If they’re just unwilling, I’ll learn my lesson that that person is untrustworthy, and will probably not have much to do with them in the future.