Is there a reason you can think of that this plugin wouldn't work properly on a multisite installation? Any given gallery will show up properly on the user's personal site, but when viewed from the main site (which shows posts from every user), it simply shows the magic code (<!–FBGallery 2508247720683110404 –><!–/FBGallery–>)
Any ideas? If you think it's fixable, my client will probably pay the $20 for your specific support, but if you don't think you have the time to do multisite testing, then they'd probably rather not pay, know what I mean?
If it normally works as expected on multisite installations, we'll just wait until the client is ready to update WP to the most recent version to see if that fixes it.