Really, nothing on the tax front for "you". But on the legal front, it's quite apparent that you are knowingly participating in an illegal activity since you are aware they are not allowed to sub-let space in the property per an existing rental agreement. If/when the landlord/property owner finds out about it, you and the current tenant can expect a fairly swift eviction. Get permission from the property owner for this. They "may" allow it. If you don't ask, then there's no chance.
