Controllers are not generally rated in watts but in amps. It is the controller that limits the power given to the motor.
A 48v controller rated at a maximum of 30 amps would give you 1440watts notional . In order to get this the battery must have a maximum delivery rating of at least 40amps so as to not be stressed and damaged by such a large draw. Such a battery will be very expensive and heavy.
A 1500w motor will be a direct drive. It will be large, obviously illegal and very innefficient at any thing other than high speed on the flat. Stop start riding or hill climbing will generate large amounts of heat, innefficiently using battery power and causing range to be very poor.
All the extra weight will require a serious upgrade of braking.
Your 'friend' has probably been given the motor because the previous owner realised that it's best use was to make it someone elses problem.
When the project is built, having spent a lot of money, to be legal the bike would have to be registered, plated, have proper lights fitted, be insured and the rider must wear a motorcycle helmet and the bike should be limited to either 25kph or 45 kph depending on what class it is registerd as.
So my advice is to either buy a motorbike or, if he really wants to build an electric bike, get a 250w geared hub.