As mentioned above your buddy will need to complete the immigration line at Heathrow anyway. If in transit, there's no fast track/priority lane back up to departures so this can be a long drawn out affair at busy times with all transit passengers using the same immigration and security lanes. A lot of posters here do just that when in transit because its quicker. If he's travelling on a non UK/Euro passport he can use the fast track immigration line which should be as fast if not faster than the UK/Euro lanes. All he then needs to do is collect his bag and head back up to departures to check in for his flight to MAN. Again he can use the Club bag drop and fast track security lanes - he just needs to ensure he's through security at least 45 minutes before scheduled departure of the MAN flight so hopefully he's booked a realistic connection.
If he has and there's time to spare, there's also the arrivals lounge one level up in the meeting point after baggage reclaim and customs clearance. The food and drink offering here is significantly better than the Club lounges in departures prior to the MAN flight.