WMM is about as hassle free as it comes for both imports/exports, but its functionality is limited, so extensive editing to either audio or video is nearly impossible. It will meld the two together fairly nicely (and yields a decent result), but you may have to import the video and do the audio edits in SONAR with both applications open. It has been a while since I have used WMM, but remember the import/export was convenient, just the guts lacked editing capabilities.
As is a collab project, you may want to agree on a timeline so you are both working to the same result (then only "assembly" of video-audio is the issue).
Edit: There are some "video heavies" on this forum that could probably do this task quickly, but would probably need a very close sync of audio and video if you happen to get into a pinch.