The hotel suggestion is a good idea. The Express by Holiday Inn is the closest and can be walked in around 6 minutes but if they do it on the day whatever direction they are coming from they should avoid the main route into the airport and head for Frank Lester Way via Eaton Green Road and then follow the sign post for the long term car park. This was the airports old access route in the early 90's and will be clear. The route isn't signposted but if they have a sat nav by putting in Eaton Green Road it will miss the airport out. Once on Eaton Green Road they just go up hill, past the Shell petrol station and take the first right into Frank Lester Way.
For a 7:35 departure I would be looking to arrive in the area around 4:35 at the very latest. Leave it an additional 30 minutes and they could hit trouble if they are coming from the M1 as the jams come from nowhere in just minutes. If they want to use the main route in I would be looking at 4:15. They should sale it at that time and wonder what all the fuss was about.
Also by arriving early the long term car park coaches should just about be disruption free. What they don't want to happen is to queue to get into the airport and then queue again to get to the terminal from the car park assuming there is a coach to catch. It is always better to arrival hours early than a minute late.