Welcome to the Forum @anon64163577 and thank you for reaching out!
Please have a look at the following guide article to learn the steps to add a second email address: How to Control Your Profile Settings in Asana | Product guide • Asana Product Guide
If your colleague has followed all the steps outlined in the Guide and continue to run into this error message I would recommend him/her to reach out to our Support Team who will be able to manually add this second email address for him.
To contact our Support Team you just need to follow these steps: How do I contact support
I hope this helps Stuart! Please let me know if there is anything else I can help you with!