In absence of a Crew Fatique reporting scheme or an Ops manual procedure, I'd suggest that, in the first instance, the crew member should complete a Company Air Safety Report. They should, of course, be taken off the roster, until fit to fly. Fatique, (rather than simple tiredness) which results in reduced capacity as an operational crew member, may point to a roster/rest pattern that needs investigation- recording of a fatique event in formal manner is important. Your FTL scheme is designed to 'avoid fatique', if a crew member who has taken advantage of the rest opportunites it provides still ends up in a fatiqued state, then someting is wrong- somewhere.
A discussion with the Company GP, however, is a prudent course of action and should be seen as a supportive measure rather than one that puts the crew member's word in doubt.