Not ideal, but you can create one repeating event for however many days she will work that month, then drag each event to the proper day; or create them for every day of the month, then delete the extras. Each time you drag/delete/change an event it will ask you if you want to change just that event, or all events. Choose just that event and it becomes a singe event on the day desired.
Best done from the Month view.
To drag, tap and hold until the event floats, and then drag it to the target date.
During my short test, events occasionally shifted back to the original date after a couple seconds; probably something to do with me getting ahead of iCloud syncing when I started moving them around too quickly.
Other than a third party app, as Hayles suggests, I don't have any other ideas.