iPhone texts going to email instead
Given what you have said, the other people must be sending messages to your father from their email, so you will need to check what the other person's address/phone number is. If he wants to text them, and not email them, he will need their phone number and not email. I suggest you add a screenshot of the contact information of your father's friends, but it is likely the other person's Contact information includes an email but not a phone number.