Why won't just ONE person get my texts?

It happened out of no where for a few days now. I've always been able to text this person fine. But now, they can text me and I'll get it, or they can call me and I can call them, but they just don't receive any texts I get. (I've seen it in action)
Btw, I have an iPhone 4s and they have the Galaxy. (Newest one or one before it, I don't remember)

So, please tell me this is a quick fix.

Added (1). Oh and I have Verizon

Pretty sure they have Boost Mobile or AT&T, if that helps.

Maybe they didn't get it or you didn't send it

Check with your carrier

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