we run a fitness studio that has PT in it so can give some thoughts as well.
Some of the trainers I know of make up to 40k. They work damn hard at it, they have a solid client base and like has been said here, they look the part.
Most of the trainers make around 8k a month with us. They maybe do sessions elsewhere but I dont believe it is a large chunk.
AS has also bee said, it can get exhausting as almost no clients listen and it is a constant battle to try get results. It is impossible to give someone a decent physique in 3 x 1 hour sessions a week, especially if they eat like pigs and dont do anything else for the rest of the week.
Pilates is going to be an uphill battle. He is going to have to compete against a mainly female dominated space and unless he owns a studio will struggle. He will likely get paid a few R100 per class and at best do 2 classes a day.
End of the day, unless he can run his own studio or gets a really really good client base and a solid reputation, he will not earn a lot.