My friend is in coma since 1 week after cutting his threads when he wakes they say he will loose his memory, what will he forget? and for how long?

uncertain: I am not sure what "cutting his threads" mean, but what someone is like after waking up from a coma is pretty variable, and only time will tell what his deficits are.

Recovery after coma: Hello. Coma is an event which prevents new memories from being made and often erases old memories. Depending on the length of the coma, cause of the coma, medications used the amount of memory loss and even recovery from the coma itself can be vaguely predicted. The longer the coma, more severe the initial cause, more medications needed all predict a poor recovery and even the possibility of death.