If your friend meets the criteria he would be able to apply for rent supplement. Officially you would be a landlord, although as you also also live in the house you would not have to register with PRTB (as i understand it)
There is no amount of rent you could charge that would not be assessed, the additional income disregard does not apply to this kind of income, and therefore every cent you get in rent from your friend will be assessed off your mortgage interest supplement.