Would be interesting to know where the OP's colleague started off from. Gambia Airways seem to have several 707s come and go in the early 1990s, including all-passenger models. They probably got various slightly grey market charter work for them, not necessarily from home base. UN relief flights (they always seem to go for the cheapest as long as your registration state is a UN member) down into Africa, Hadj flights to Jeddah, odd large consignments for the freighters, stand-ins for European IT operators, etc. They seem to have been periodic visitors to Manston for freight and for maintenance. The one described above as still having Mauritius livery lay there from 1993 being slowly robbed for parts until there was nothing left a few years later.
Would the crews be principally South African expats ?
Depending on how aviation-knowledgeable they are, if they went from London Gatwick I might also start asking about the various 757 IT charter operators running at this time to Banjul, a market which seems to have come and gone. I've had 757s misidentified as 707s before (which in the cabin it effectively is). Not everyone is like us lot !