My friend is in an insulin overdose coma, will he be okay when he wakes up?

Serious injury. The brain relies on glucose for metabolism and developing a coma as a result of an Insulin overdose is very serious. Their recovery will depend on how long glucose was low and other factors including the presence of prior vascular disease in the brain. There could definitely be long term impairment as a result.
Coma recovery. Recovery from a hypoglycemic (low blood sugar) coma resulting from Insulin overdose is variable. The recovery depends in part upon the severity and duration of the hypoglycemia. It also depends upon patient factors such as age, with younger, otherwise healthier patients having better recovery than older patients.