The premise in your question suggests they are letting out their owned property.
As someone helping on a tax return you are probably more knowledgable than me but the following come to mind...
1. If they started renting before 7 dec 2010 they can claim relief on the rent they are paying.
2. the MIRAS they are still claiming is really quite small and will decrease further.
3. Does not the fact they have ,in effect, an investment ptoperty mean they claim 75% of all the inetrest as an expense?
4. But will interest on the loan increase as it is now not a home mortgage? there are many posts on this last point. It depends on loan conditions in the agreement.