It depends. When you say "user", do you mean you created a user account for her on your Mac and that was backed up to the Time Capsule. If that is the case, all the files will be contained in one sparse bundle on the TC and their is no way to remove just hers. You would need to delete the entire sparse bundle backup from the TC and start over.
Now if she was using her own machine on your TC, there will be two sparse bundle files on the TC. Just figure out which is hers and delete it from the Finder.
It will look like this screen cap.
Let me know how it goes. I've tried deleting sparsebundles backups from my Time Capsule in the past and its so excrutiating slow (almost no progress at all over long periods of time) I usually end up erasing the whole drive and starting new backups.Thank you so much for replying - I only remembered about this thread now, many months later! I'll give it a go in a moment - I'm sure it will work. Thanks again.