As a regular at Manchester Airport (taxi driver) I can tell you that pick ups have to be from car parks except maybe terminal 3.
If you come off the M56 at junction 6, go right at roundabout and then left at traffic lights, first right is Avro way. There are two long parking areas on the road where parking is limited to 30 mins but no-one patrols it. Or you could park in the Premier Inn over the road (two hours free but get ticket from machine) if your motorhome will fit a bay.
Get your friends to phone you when they have their baggage then go to the airport.
Tell me which terminal they are returning to and I'll give some suggestions.