I am not questioning that this was your firends experience but my recent experience is completely the opposite. I arrived around 6pm ish on a Cathay flight from HKG last Thursday and literally had 1 person in front of me.
Leaving on Sunday at 1pm the wait was approx 10 mins.
I think it depends on when you arrive. If you get into BKK around 3.30pm as all of the Euro flights arrive you are bound to hit a lot of traffic. Similar story if you are leaving mid to late afternoon.
On arrival I always move to a different immigration hall if the first one is busy this has never failed in reducing the time I have to wait.