transferring mortgage between 2 properties

Hi,
up to 5 months ago a friend was living in his apt and renting out spare room under the rent a room scheme. This apartment has no mortgage on it. 5 months ago got married and moved into new house. THis house has mortgage on it, he is now renting out the apartment.

i suggested he should transfer all or some of mortgage on his house to the apartment so the rental income on the apartment would only be taxed after the interest portion of the mortgage would be deducted...
any suggestions on likely tax implications of transferring mortgage?

He will not obtain a mortgage interest deduction for the appartment by tranferring the mortgage.
Interest is only allowed where it is incurred in the purchase or improvement of the property.

It is not what the mortgage is secured on but how the funds were used that matters. In this case the funds were used on the purchase of what is his principal private residence. He will get TRS on that subject to the normal limits. The apartment has no finance on it for its purchase or improvement and accordingly no deduction can be taken.