I wouldn’t text, it invites a response or at least opens you to rejection.
Send her a card, saying how sorry you were to hear she was ill. Get well cards don’t necessarily result in replies but would certainly show you care.
when I learned my Aunt was dying of cancer last year I wrote to her with memories of the great times we’d had together. She’d cut me out her life 10 yrs ago but I learned that she loved the letter. I never got a reply, but that wasn’t what I wanted, I wanted her to know she had done some lovely things for me as a child that I’d greatly appreciated.