Pigs who lose their cagemates can become depressed, depending how long they lived together and how strong their bond was. She may stop eating, she may have a lack in interest in things, she may sleep more. If she stops eating, take her to the vet, or get her another cagemate as soon as possible. If you decide to take her to the vet, they might recommend syringe feeding her mushed up pellets or Critical Care. Not sure what else they'd give you. The sooner you get her another friend the better. Don't wait and see if she'll be effected. Look at all the possible shelters and rescues. I know it may seem like getting another pig means you're "betraying" Girl, but you're doing what's best for Nike, who is fortunately still with you. Girl will always be in your heart, I'm sure. It sounds like you cared about her and loved her a lot, and I'm sure she knew that
The new piggie won't be a replacement of Girl; she/he will only be a new friend for Nike and a new beginning for you.