An apprentice being forcibly 'failed' by their parens would not generally be willing however, and a tytalus is unlikely to have either of those mysteries to initiate.
The Path of Strife method doesn't require a willing subject, compelled via ReMe is fine. And the Tytalus could possibly have convinced a Path of Strife Magus to do it. Or maybe the Tytalus did some original research into creating PeVi rituals to destroy the Gift, using this guy as his test subject on an effect being made for insight. There are lots of possibilities.
It states "A magus of strife cannot sever the Gift, except with the active, knowing assistance of the Gifted individual."
Yes. I don't see "willing" there at all. ReMe! "We're going to severe your Gift now. Work with me." You have active a knowing assistance.