When it doesn't scan and they ask to see photo ID, as they do most of the time, if you're lucky they'll just make you buy a ticket. If you're not lucky they'll cancel your friend's AP and make you buy a ticket. I guess if your friend's AP is about to expire he may not care.
If an AP holder doesn't have photo ID they might wave him through or they might ask questions like when and where you bought the pass, when did you last use it.
If you're on a tight budget do some of the timeshare tours for a free pass.