My friend forgot to uninstall her Whatsapp on her previous phone which was bought buy someone she doesn't know. When she try to use her whatsapp using the same number, she got a reply from the person who bought her phone. The person doesn't know how to uninstall the whatsapp. Any solution to remove that person from using whatsapp with her number?
Please tell your friend to ALWAYS DO A FULL WIPE of the device before to sell it. This is very important for your privacy!
Anyway, please refer to the official whatsapp FAQ to solve your problem: https://www.whatsapp.com/faq/en/general/24460358
If you are not ready to activate WhatsApp on a new phone with the same number, we can deactivate your account. Email us the phrase "Lost/Stolen: Please deactivate my account" in the email body and include your phone number in the full international format as described here. Please note that we are not able to help you locate your phone. Remote WhatsApp deactivation from another device is not possible.
Don't sell phones that you've used. Melt them down. Otherwise you are selling your personal data which can easily be recovered.
What she needs to do is install whatsapp on a new device using the same number that her old device had. That will lock the whatsapp on the old phone and force the new owner to put his/her new number in.