Those are Malaysian trumpet snails which are pretty harmless (in fact many people like them as they do a good job of sifting substrate and eating leftover organic bits). To decrease their numbers your friend can decrease the amount of food he feeds his fish and maintain consistent maintenance on his tanks (specifically gravel vacuuming and algae removal). There are also a number of DIY snail traps available (just google DIY aquarium snail trap) that are very effective. This is an aside note but 200l is too small for clown loaches so hopefully he's planning to upgrade them to new homes as they get larger.