It does sound like she would benefit from a friend. Will you, once she's done her heartworm treatment, be taking her to classes or anything for socialization? She sounds like she'll need a lot of "rehab" - occupational therapy for being a dog. Perhaps greyhound rescue websites would be helpful for you, as she, like many greyhounds, has always been caged ... they have tips for overcoming fears.

I'd be cautious and introduce her and any potential new dog on neutral territory. Once she's done the heartworm treatment, her energy level may rise, too. I hope she will be okay!

Do you have any dog-owning friends nearby that she could do "play dates" with?