With the phone number, it will be possible to retrieve the email address and reset the password, so it is best to use the contact info from someone you trust.
Yahoo will prompt him to confirm the name and surname before sending a code to retrieve the email address. To reset the password, the code will be sent without any verification. Thus, you agree that it is not a strong protection to protect unauthorized access for the number's owner, but of course, it's normal.
The good news is that you can use your own phone number. Even if you already have a Yahoo account, creating another one with the same number is allowed. If you exceed the limit of accounts, you'll get an error "Your mobile number is listed on too many Yahoo accounts".