It's a bad idea to get a friend for your female gerbil at this point because introducing adult females to anyone but an adult male is very difficult (they are very aggressive) and you wouldn't want to introduce a male, unless he has been neutered, because they will breed prolifically.
A friend of mine has had a lone female gerbil for years now (the pet store should have insisted that they buy two females from the same litter but didn't) and she is just fine on her own because she gets alot of attention from her owners. Another option is to have a separate cage that houses other gerbils – gerbils can sense that there are others nearby (but not in their territory) and that comforts them as well.
A friend of mine has had a lone female gerbil for years now (the pet store should have insisted that they buy two females from the same litter but didn't) and she is just fine on her own because she gets alot of attention from her owners. Another option is to have a separate cage that houses other gerbils – gerbils can sense that there are others nearby (but not in their territory) and that comforts them as well.