There was multiple legs without long breaks. Iirc, the co-pilot suggested he go since he was holding it since the last stopover.
How long does it take to take a whizz? I walk roughly the same distance from my desk to the restroom and I’m back in my chair in 4-5 minutes. It takes longer than that during a stopover to let out and let in the passengers.
Unless the co-pilot knows the pilot well enough to know he always takes his potty breaks after take-off, the strategy of spiking his drink doesn’t make much sense.