Shrooms are natural and therefore the dosage is completely unpredictable. He bought these shrooms from a new dealer so they absolutely could have been more potent than others he's bought. Or maybe all the past ones he bought from a previous dealer were super weak. Reality is that since its unregulated and if you're buying from different people, you will never know the true potency of the shrooms you're taking.

Also I've had super delayed shrooms trips because of eating them on a very full stomach. It takes longer to digest because of that. But I also agree with you that these shrooms could have been laced with something else. But I wouldn't discount the fact that these might have just been super strong shrooms.