If a new client is having hard time verifying his/her email, then, as an Admin you can verify them.
All you need to do is go to the CUSTOMERS tab of Appointy admin and search for the respective client in the SEARCH BOX (by email id/first name/last name) and when the search result comes up, just click on that client.
After clicking on the client, you will see the client's details. Locate VERIFY link (see the screenshot below), click on it to verify the client.
If you need any assistance, we are always there to help. Please reach us at firstname.lastname@example.org with complete details of your concern, and we'll be happy to assist.