Advice them to:
1. Delete FB from your phone and stop using it all together.
2. Delete Twitter from your phone and stop using it all together.
3. Use tor browser or this is also good -
https://duckduckgo.com/
4. Stop using Amazon on your phone.
5. Always log out and close all apps/programs when not in use.
6. Same applies to all other so called free services like LetGo, Whatsup, Skype, etc.
7. Turn off any location service on your phone.
8. Trash all home assistant devices you have - echo, google, etc.
9. Anything that has the word "smart" in its name or description is a potential tracking device.
There is no free lunch period. You either pay cash for it or with your personal/financial data or both.
They want to be forgotten - then they need to live in the 80s era when TV was TV and phone was a phone to make and receive calls not to track every move you make and every thought you have.
With that said as far as XC is concerned you can simply delete their account and orders and customer info is gone.
But if you use GA, FB, etc to track revenue, purchases, clicks, you need to do whatever needs to done from there....