Strokes are rare in guinea pigs, but can occur because of heart or brain disease, brain injury, infection, stress, toxins, or high blood pressure. If infection was the cause, the vet may advise against introducing a new guinea pig because of the risk of cross-infection. Alternatively, it might be the vet thinks the remaining guinea pig would find it hard to accept a newcomer. I would suggest speaking to the vet to clear this up.