I have a area of rot on a window frame (image attached).
I hacve scraped it out and will let it dry for a few days then intend to
use wood rot hardener then fill with 2-part epoxy stuff.

Is this the best way to go or should I cut a section out and replace the
wood. Damaged area is approx 20cms long and approx 2cms at widest.

Other question that piece of the sill does that go through to the
interior of the frame? Not sure how these things are constructed.

https://imgur.com/BXRr3bd