Was she the dominant pig or the subordinate pig in her last pairing? I've never been in a position to bring my surviving pig along to choose a friend either. However, I have tried to pick a friend who seem to fit into the existing power dynamic. Linney was a VERY bossy dominant pig, so when she lost her companion, we made sure to choose a pig who was really passive because I knew that she would not co-exist with another bossy pig. When we had to match up Sundae (the very meek pig), we figured she would probably be subordinate to pretty much anyone. The pig we picked (Hadley) seemed to be the upper end of the pecking order in her cage. She ended up being the boss of the (much larger) Sundae. Then when we had to find a friend for Hadley, I was kind of stumped, as Hadley was the boss of Sundae, but a flower pot could have been the boss of Sundae! Sundae had no desire at all to be the boss. So I didn't know whether Hadley would fight to keep her top status if we got another pig with a stronger disposition. As it was, we ended up with Leela, who we suspected was not top dog in her existing group (as she was hanging out in the back corner.) Leela turned out to be REALLY good-natured and was fine with letting Hadley be the boss. So if you have the opportunity to see prospective pigs interact with each other, I would lean towards choosing one who appears perhaps similar in temperament to the pig you've just lost. Especially if the remaining pig is dominant and has a strong personality, I would try to avoid another forceful personality.
I did not end up getting another piggy today as planned. They said there was a chance the piggies had ringworm or were exposed to it and I was too unsure about it to risk exposing Wiley 
