Did it many years ago myself.
Herself is rubbish at finance. She had credit card debt, personal loan, overdraft and a good salary.
Bank would not consolidate. We were only a couple for about two years at the time.
Drew up a standard loan document with a 3% interest (deposit rate at the time) and she paid it off over 4 years.
Once both parties know and understand that the transaction is totally independent of any relationship, then it should be fine.
As deposit rates are currently zero, a zero interest rate would be appropriate. Hence no loss or gain.