I think that this person is not very good at spelling. The correct form is:

Assez

Spelling shortcuts are common in textos/SMSs, and one that will require less characters seems to be quite welcome in general. So one could think assé could be a quicker way to spell assez, but it is likely not the case, because typing an accentuated letter on most touch-screen devices takes longer than typing two letters (hold the finger for a moment on the letter you want an accent on, then slide the finger to select the accent you want, then let go).

Most devices also have an auto-complete option that will let people choose assez, which again makes it quicker to type than assé.

Also, assez is common enough a word for someone to know its spelling, unless one actively wants to disregard it, for a stylistic purpose or other.

I haven’t really been exposed to assé, and I suspect it is rather rare, even in informal texts. It really doesn’t have much to promote itself: it is incorrect and longer to type, and it’s not even introducing a possible alternate pronunciation.

On the other hand, as pointed out in the comments below, one might expect AC as an abbreviation of assez in SMS.