Perhaps tell friend that London Euston to Manchester Piccadilly takes less than 2h10 by train and goes every 20 minutes. Then ask friend whether he/she (and other people) would really pay for flights instead
This suggests more that flights from LHR-MAN are more redundant than flights LGW-MAN.
LHR is closer to both MAN and London Euston, so why fly? LGW is further than both MAN and London Euston...