I'm always extremely dubious when I see claims of people having been spiked with rape drugs. It's effectively an urban myth, as numerous studies have shown the prevalence of it is minuscule. It does happen, but the rate of people falsely claiming they, or people they know, have been spiked is massive.
http://www.telegraph.co.uk/news/uknews/crime/6440589/Date-rape-drink-spiking-an-urban-legend.html
A major US study with over 1100 people self reporting that they'd been spiked only found this was true in 0.5% of the time. Predominantly, people just had elevated alcohol levels.
A three year study in the UK at a hospital tested people coming in saying they'd been spiked. Results were the same as the US, with only 2% having a positive result, the rest were just drunk or had mixed booze and recreational drugs.
So the dude in question could be part of the 0.5%-2% where what he's saying is true, but far more likely just drank too much or mixed his booze with prescription or recreational drugs.