One friend does not receive my texts on his iPhone, but is able to on his iPad.

Is it possible that you may be sending your friend texts using his/her email address instead of phone number? Your friend's iPad may be set up using the email address and the iPhone with the phone number.

If you find that you are using your friend's email address then delete the entire text thread and start new phone by typing your friend's number instead and send the text.

Axel F.