Newuser, it is wonderful that you are thinking of your piggy friend and looking for some company for her. You are correct, they are herd animals naturally and really do better with a friend.
If you are looking for a friend for your girl at a rescue--which I highly recommend--the people there may be able to guide you based on the personality of your pig to make a good match with an adopted pig. You may even be able to find a neutered boyfriend for your girl at a shelter.
Whatever you do, make absolutely sure that your piggies are correctly sexed before you put them together. You girl is too old to have a first litter of babies, it would probably kill her.
When you bring your new piggy home, put their cages next to each other--or if you have a big C&C cage, add some cubes to divide it into two apartments. Let them see, smell and make noises at each other for awhile.
When and if you think this is going well, you can try them together, supervised only, and see what happens. I brought my girls in at different times, and everyone got along fine. However, if you have a super dominant piggy, they may decide they want to be fence buddies and never actually live together. They will still both get a benefit from being next to each other, knowing there is another piggy there, even if they can't actually live together without fighting.
There is a great site I recommend, it will have tips on everything from new cage size recommendations to advice on introducing your piggies. Here it is: www.cavyspirit.com. Another is www.guinealynx.info.
You can find more suggestions on introducing the new friends there.
Best to you and your pigs!
PS Sad about your friend's pigs. They should have been separated as soon as bullying was noticed.