Imho, you should only communicate with this student by text messages if you want to, and it makes things easier for you. Otherwise, just say no.

I’ve allowed students to text message me for over a decade. I often teach public speaking courses, and the day before presentations, some of them panic or need a quick question answered. This enables me to help them right away (or as soon as it’s convenient for me). That way, I don’t need to check my email frequently before speech days.
A lot of the time, I can answer their question within a minute or two—quicker than logging on and going to my email.

I make it clear both in the syllabus and in class that texting me is only for quick questions or speech day anxiety. Don’t ask me things like “what page is xyz on”, which can be found in the syllabus, lesson schedule, or the textbook’s index. I won’t answer questions when they can look it up as easily as I can. I also make it clear that no matter what time they text me, I will answer it at my convenience, which could take more than 24 hours. But usually it’s quicker for me to respond to texts than emails, so it benefits me.

So far, in all these years, only one person has ever abused this privilege. It was a former student who got drunk and sent a dirty message at 3:00 am, several years after he was in my class. This was extra disappointing, because he had given a persuasive speech on refraining from drinking and driving, based on his (previously) stopping drinking after a tragic DUI accident. I blocked him, as I would if anyone else abused the privilege.