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!?

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.