If one person wants to leave a joint mortgage, there are a few solutions. The remaining co-owners can buy out their share, removing them from the mortgage entirely. This requires agreeing on a price.
Renting out the vacant room(s) is another option, though you’ll need the lender’s approval first.
Finally, selling the property and splitting the proceeds amongst co-owners is a clean break, but requires agreeing on a selling price and factoring in estate agent fees.
It’s wise to consult a mortgage broker for tailored advice on the best course of action.