How are you trying to access the share from your bro's machine? Are you just manually typing \\machine_name\share in the address bar of Explorer? If so, what happens if you temporarily map a network drive and specify the username and password (using the credentials from your machine and not one of his accounts) - do you still get the same problem?
__________________
Just re-read your post and saw I got it round the wrong way. Instead of doing that on his machine, do it on yours and enter the username and password for an account on his machine when you're mapping the drive (under the 'Connect using a different username' option).