Yeah, my males chase my females around all the time. And yes, it is part of the mating prosess. How long are you going to be looking out for the fish? If it's just a short period of time, like a few weeks, then you might want to get a breeder new an put eather the male or the female in it, then swich them every few days to minimise stress. Get the largest breeder net you can find! If you'll have the fish for more then a few weeks, you should try to get a tank divider or someplace else to keep one of the fish. A simple 1 gallon tank would be enough for just 1 guppy, though it would get lonely. And if you are going to keep the fish for a long time, like several months, why not get into the hobby of breeding them? True, it can be expencive at time, but once you get the tank(s) set up with filters, heaters, and some kind of aeration, then the price is reduced to the cost of medication and filter replacements. If you do get into the hobby, do some reading first. I can help you out some too, just send me a message!
I really hope that helped!! Good luck with your friends guppies!