I presume MC will update library to tags or tags to library only according to which one has changed. Hopefully, someone who actually does this sort of thing will confirm this for you.
Regardless of how it works, it would be a good idea to turn off automatic updating until your two-machine tagging exercise is complete. What I would do is tag a group of files, then select them and
Update tags (from library). Then select the same group of files on the other machine, and
Update library (from tags). If the information you see on both machines is not then exactly the same for those files, then something is very wrong. In other words, you're wise to stop. You should not proceed with tagging until you understand what's happening, you're sure this works, and you have confidence in what you are doing. Afterall, there's a gf at stake.
I went to her laptop chose a file... 5 starred it, and renamed it, then went to desktop REimported library, went to artist mode and artist\album song etc for that SAME title and that same file tag and the rating hadnt changed!
I'm puzzled. Why would you want to rename the file and re-import it? Isn't MC (on the desktop) just going to see that as a new file and import it accordingly? That doesn't prove anything. If you looked at the old entry for that file, it would not have the new rating—because the old file didn't change. If you updated that entry, it would tell you the file no longer exists. If you looked at the new entry, then you should have seen the new rating, unless you did not update the tags after making the change.
if i update from library on both machines wont that cancel out each others work?
You are using MC on two machines to update the information
saved in the media files themselves. If you
Update tags (from library) for all the files, then, yes, you would overwrite whatever updates were made on the other machine.
So don't do that! Just update the files you changed. And don't change any file names while you're doing this tagging! You're counting on being able to exchange information between the two machines/libraries based on those file names. MC might be able to recover from such nasty tricks, but you're asking for trouble.
Once you're finished the tagging, both machines/libraries will be able to read the same information from the same files. You can then rename all the files (to reorganize them on disk) using the desktop/library. I'm not sure on this point: MC might be able to update the new file locations in the laptop/library, but I would be inclined to rebuild a new library by importing the files again after they've been renamed.