Your friend needs to ask to see the parents of the pig she wants to buy. People around here always advertise mini or teacup pigs, but their parents are huge. If they won't show you the parents, don't buy the pig unless you want a larger pot belly pig. You can control some of their growth by feeding them right, but a lot comes from the breeding, too.
I breed and sell Pot-Belly pigs. I have one boar who weighs about 65 lbs., one that weighs about 50 lb. My sows range from 35 to 60 lb. They range in height at their shoulders from 11 inches to about 16 inches. They are every bit as wide as they are tall, though.
The piglets are really small for a long time. I have one left from the last litter that is almost 2 months old and she is only about 4 inches tall.
Some of my pigs are small compared to a lot of them, but I do not advertise them as mini or teacup. Mine are normal sized pot belly pigs.
You do have to feed them correctly or they will get up to 150 lb. easy. They really only take about 1 1/2 cups max. of feed per day. They need all the grass and hay they want, and lots of room to exercise to stay healthy. My pigs are in about a 1/4 acre pasture and they run all over it every day! They love toys, DH put some plastic barrels in their pasture once just to get them out of our way, the pigs had a blast with them. They pushed them over and rolled them all over! It looked so funny to see them pushing those 50 gallon barrels around.
ETA: And if you read the fine print on mini pig websites, they say the normal weight is 50 lb.
Jean