A mate eh?
I've not been the same since my heart operation, I blame the tablets. I'm on beta-blockers and ACE inhibitors. The beta-blockers are to stop me getting excited and the others are to keep my blood vessels dilated. If you, or your mate, are concerned then what's wrong with a trip to the doctor? There is one test to give an indication of whether it's physical or psychological. Most men get an erection in the early morning. If you take a strip of stamps (don't have to be first class:)) and wrap them around the base of the object in question and lightly gum them together then if you have had an 'event' during the night the perforations will have been torn.
Good luck