Interesting post. Unless your friend has deep pockets to spend money on a flight to check out a vehicle, I would suggest an inspection done by a professional. Simple to set up online and you receive a full inspection report. Cost should be nominal at about $100.00-125.00 depending which company he chooses.
Price is up there and for that amount, vehicle should be in very good shape, running and all issues such as fuel, hoses, a/c, timing chain, etc. should have been addressed. Also, for that kind of money, service records should be available. I find that besides checking the mechanical side of the w108, rust is a big issue to look into.