So they are receiving MMS but not SMS (pictures but not texts). The question is, what the "Samsung Messages" doing? SMS are SMS regardless, and the SMS app is just an interface, the service behind it is the same whatever app sends the SMS. Hence if messages work from the Samsung app but not from Textra the obvious suspicion is that the Samsung app is using a different protocol for this person. Not having a Samsung I don't know what options that app has, but I can't help wonder whether it's somehow sending its messages as MMS (maybe as a fallback to RCS "Chat", which Textra doesn't support but I think Samsung's app does)? You could however try another third party SMS app: if it also doesn't work from a different app that will increase the suspicion that the problem is with SMS rather than Textra and that the Samsung app is doing something unusual. But I'm afraid all of this is pure speculation.

Are there any other reports of such problems with iPhone users? It would not be unfair to say that Apple's message app has a chequered history as far as messages from non-Apple devices goes, so I wonder whether the iPhone might have received an update shortly before this started which could have messed something up? But again, that's just speculation.

Just to cover all bases, it might also be a carrier problem. I don't know whether they can see what's going on, but it might be work asking them.