It depends where you are to some extent. In the US you have bugs that do eat Yew regardless of what it says in the books... borers can't read!
IMO you should cut out absolutely ALL rot and have no expectation that even then the rest of the wood will be any good.
Sapwood with rot in it is worse than useless.
If in doubt, saw off the doubtful wood and get down to good clean heartwood, which can be used backed with hickory Maple Boo etc.
I've wasted time on wood that was doubtful, listen to that inner voice that's whispering "it's rotten" in your ear.
Del