You can get the SID for the user and then look it up under HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList and get the ProfileImagePath value.
Once you have this path, you can get CLSID_LOCAL_APPDATA for your user, convert the absolute path to a relative path to your profile and then append that relative path to the other user profile path.
However, keep in mind that this is relying on an undocumented registry key and can break in future versions of the OS. (Or, as Raymond Chan would say: "Now that you know how to do it, let me tell you why you shouldn't do it this way..." :-))
If you have a token representing the user, you can use the SHGetFolderPath or SHGetKnownFolderPath (on Vista and up). However, there are certain security restrictions and you should read up on MSDN for details.
SHGetFolderPath - http://msdn.microsoft.com/en-us/library/bb762181(VS.85).aspx
SHGetKnownFolderPath - http://msdn.microsoft.com/en-us/library/bb762188(VS.85).aspx