They must upload any official document confirming they divorced.
If they do not have any official documentation, they must upload a covering letter confirming the reason they do not have the divorce document and a letter signed by a professional person.
If your parent is currently single, they can upload a covering letter explaining this and evidence of their single status, such as a Tax Credit award letter or Council Tax letter showing single occupancy status.
If your parent has since remarried, they can upload their new marriage certificate and a covering letter to explain.