Are you both using the MP version?
If your opponent doesn't and you do, then he wont be able to load the save.
Only reason I could think of for failure to load save (apart from file corruption).
File corruption was the problem, something neither of us had considered, and after going round and round in circles chasing our own tails, and trying all sorts of different things, the answer was there all along.......
Man, I feel so dumb....
Our lives may be more boring than those who lived in apocalyptic times,
but being bored is greatly preferable to being prematurely dead because of some ideological fantasy. - Michael Burleigh