iPhone not receiving texts from one person

I’m having no trouble receiving texts from anyone on iMessages. I’m having no trouble sending texts to anyone except for one person. We have tried every single thing we can think of but I still can’t send a message to that one person. I can receive their texts fine. When I try to send just this one person a text, my iPhone thinks that they are sent, makes a happy noise, but they are not received on their phone. What can I do? Is there a way to reset or something?

[Re-Titled by Moderator]