without a picture no one can really give you any good advice as to whether it needs to be repaired.I am not able to access this area to get any pics. This piece of wood runs in the same direction as the plate but I can't imagine the roof trusses were attached to it or else the roof wouldn't be standing anymore. Maybe it was a shoddy repair that someone else had done by nailing it onto the plate? I hope that may help you in trying to identify what part I'm talking about.
I would like to add that if its a manufactured truss you will want to make sure an engineer signs off and approves whatever repair is attempted.
If he is working on the fascia and notice rot on a truss it is most likely just the tail of the truss(again no one can say for sure without pictures) if it is just the tail of the truss you could cut off the tail entirely and frame in a new tail to replace it without effecting the integrity of the truss.