"Structurally safe" is a loaded statement/query. Timber framing systems are fairly resilient, and can stand a fair amount of creep, deflection, etc. before they fail. It's going to depend on what load is directly above, and how often loads cycle across it. @Lurker17 has a good fix: sister a couple pieces of 2x4 onto the side of the scorched joist, push them up against the exposed subfloor ends to secure bearing and nail/bolt them to the existing joist. Add another sister to the opposing side just to make sure. But definitely get the water issues taken care of first.