Hi there, and welcome to the forum!
I think that the best thing to do, is let your pig pick her own friend! You're probably right in thinking that she'll enjoy some other piggy company, but like people, not all pigs will get along! This obviously minimises the risk of you taking on a pig, them not getting on, then you having to go to the trouble (and expense!) of housing two guinea pigs, who then will each need a friend!
A lot of rescues will now offer a dating service, where you can take your pig along to meet some potential friends, and they can be bonded by someone at the rescue with experience supervising the whole thing. Some rescues offer "speed dating", where they meet, spend the day together and go home later on, and some offer "residential dating", where your pig can stay with their new friend over a couple of days, to really cement the bond between them.
I've done two lots of residential dating now, and I'd never go any other way apart from an experienced rescue now, when it comes to pairing piggies up!
Unfortunately, not all rescues are as reputable as they make out. Assuming you're in the UK, take a look at this link
http://maps.google.co.uk/maps/ms?msid=209994852075231951564.0004b8fd9391b4257d8eb&msa=0 to find a map of all the Piggy Bank approved rescues in the UK so far. These rescues meet minimum standards in terms of welfare and rehoming and come highly recommended.
The Piggy Bank is still growing, so not all rescues that meet the standards are on there yet. If you come across a rescue in your local area that isn't on the map yet, you can ask here on the forum for peoples' experiences. Chances are someone, if not several people will have dealt with them, and will be able to give you some feedback.
Hope that helps, good luck in your search for a new piggy friend!
Keep us updated!
