I’m having trouble sending iMessages to one user.

I’m having trouble sending iMessages to one user. We both have iPhones (6s) with iMessage on and cell service. I tried restarting, turning iMessage on and off, waiting, and resetting settings. It started at some point last night when I was in the car and chose to send some texts as text messages but now it will only send text messages to this specific person by default. I can send iMessages to other people. my phone isn’t very old so I’m not sure what’s up.

thanks!

[Re-Titled by Moderator]