The pet store owner is absolutely wrong, but this is advice I have heard given out in pet shops too (the idea that if a pig is alone they won't accept a companion or that if both pigs aren't babies they won't accept each other.)
The truth is that most, if not all, pigs will be thrilled to have a companion, provided their personalities are compatible. This goes for pigs who have been alone, pigs who are different ages and haven't met before, etc. I've done introductions between sows and have seen how thrilled they are, after a period of time alone, to see another guinea pig. I will never forget my first pair of pigs- one passed away from a dental infection quite young, and the other pig, Linney, was alone for a few weeks while we found another pig, did a quarantine period, etc. Linney seemed 'fine' on her own, but the first time we brought her new friend near enough to the cage that Linney could see/smell her, a huge change came over her- she popcorned, was climbing the bars, etc. It definitely showed me how desperate she was for companionship even though she appeared 'fine' to our eyes. I'll never have a single pig based on that experience alone.
So yes, your piggie needs a friend, and before you get one I suggest you go over the stickies we have about introducing guinea pigs so that you'll know what to expect and won't make any of the common mistakes that can complicate things. We usually do our introductions by putting towels down in the bathtub, putting some hay in the middle, and a pig at either side and letting them come to each other. It works well because it's neutral territory, there are no real corners to be backed into, and it's contained and easy to us to supervise. We let them find each other. Once they've accepted each other, we make sure to scrub down the entire cage to remove any previous scents so that the pig living there won't feel protective of the space before moving them in together.
Make sure to be positive of the sexes of both pigs before you introduce to make sure there are no unplanned babies. I promise you that you will see that two pigs are happier than one- they interact with each other, they can express their natural behaviors/communication in a way they cannot do with just you, they will feel safer and more secure, and you will really get to see their little personalities shine.
do I introduce a new friend or not? She seems very happy by herself and flies around her cage when I sit next to it.
