When you go in to the assessment you want to edit, you must first select which group you want (upper right corner). Then, you make your edits and save. Now if she is in a group with other students then it will change it for all students. Best bet is to have them in a separate group.
I believe changing groups only allows you to alter the settings for each group. If you change a question it will affect all groups. You would need to duplicate the assessment. Use your group setting to make the duplicate visible to only this student (and the original invisible to this student). You can make any changes you want to the duplicate.
I use this quite often and you can alter it as long as you change the group before hand
There is a prompt in the question editor which notifies the user that deleting a question when filtered by a group will delete it for all groups.
If a user wants to create an assessment specially for some group, the best practice is to create a new assessment for that group and add questions to it rather than deleting questions from an assessment that the rest of the course uses.