The source would have to be connected to supply adequate to drive the relay. If his relay loses 5 volts at rated current, then he has to have at least enough supply to support that voltage. If his pMOSFET has an on-resistance of 100 mOHMS, and his coil current has to be, say, 100 mA, then he won't lose enough from a 5-volt supply to be missed. After all, it's the current that does the work.

As for driving the gate ... if he provides a resistor divider at the gate that limits the 5 volts to 3V3 at the port pin, which I'm sure he can do, then his only issue is the gate capacitance. Since the coil current is likely to be small, he can choose a transistor that has relatively low gate capacitance, and will suffer little in performance. The resistor values will have their impact, though.

RE