Turn off iMessage for individual contacts

You can try the following-

1. Delete the contact entirely
2. Delete the ongoing iMessage thread that you have with your friend
3. Reboot your iPhone
4. Set up the contact of your friend and without the email address. Just the phone number.
5. YOU start a new SMS message thread in a fresh window

See whether the above takes care of the issue.

Axel F.

Good Point. If your friend sends you an iMessage from the iPad then it will come to you as an iMessage but then it should be in a separate window because you do not have your friend's email address in the contact.

What you can do is create another contact and name it something like "John's iPad" and ONLY list the email address. Then you will for sure have two separate windows. Make sure that, if you do this, then the contact name is not the same (so add iPad in the end).

Axel F.

By the sounds of it this person never de-registered their phone number from Apple's servers for iMessage. Ask them to go to this link and submit their phone number:

https://selfsolve.apple.com/deregister-imessage/

Once they do that, they may need to sign out of iMessage on their iPad and then sign back in.

On your end, once they've done this it should clear up. If not, you can temporarily turn off iMessage on your iPhone and then send them a text, which of course will go out as SMS and may "teach" your iPhone to do that from now on. Then of course turn iMessage back on and try again.