So I invited Biff into my crew, talked to him again near Bengar's Farm so he headed down towards Khorinis...

Then I walked back into the Valley of Mines and his corpse was laying right there!?

Sounds plausible.

Biff seems to be buggy. If you hire him, he often disappears only to reappear a long time later demanding the half loot promised (if you chose that option). I wouldn't be surprised if you found him by the ship later. Some of these bugs can be funny. I asked Diego to go with me then misunderstood a reply so told him I didn't need him (after he changed into his "Shadow" armor). So I immediately asked him again so he again said wait a minute, left, and came back in his underwear then walked down the street.