Peronally I wouldn't segregate her as the vet has said it won't be the contagious type. They may sometimes abort one lamb and keep the other, but the additional stress of being on her own (albeit with friends the other side of the fence) is, to my mind, unnecessary, unkind, and could make her more likely to abort the other foetus as well.
Yes it's sad, but there is usually a reason, and even if she doesn't have another lamb in there which she carries to full term, she is only young and (as long as you don't let her get too overweight next summer) is very likely to go on and breed normally for the remainder of her life.
Experienced sheepkeepers will tell you, "Don't dwell on the ones that didn't live, enjoy the ones that do." Much easier said than done, I know.