"Later" means on the same day, as the colleague felt persistent itch and his skin got red locally.
I believe he had cleaned the drop away with a cloth but not rinsed, or maybe just with a cloth and little water. Correct washing would have needed to exit the cleanroom in order to remove the clean clothes, then put again the clean clothes to reenter the clean room, what he definitely didn't.
"Clean the bone" is what I heard thereafter, not necessarily correct. What's clear is that he has a surgery scar at the wrist since then. Maybe the surgeon soaked the flesh extensively.
So
- skin contact with HF is definitely undesireable
- immediate proper action would have made things easier
- from one drop and thanks to surgery, the consequences were limited
- he went on using HF and so did we.