My vet always does blood work and an EKG on senior kitties before any scheduled surgeries. That doesn't guarantee something won't go wrong of course, but it does allow the vet to pick up any potential problems ahead of time. Your friend should check with their vet to see if they do the same.

Abbey

If your cat's Hyperthyroidsm is well controlled and he has been thoroughly checked out for a heart condition (a common side effect of having hyperthyroidism poorly controlled for a lengthy time), then he should be no more at risk than any other aged cat, as long as blood work is normal.