She's less than 10 years older than me, barring genuine learning difficulties I find this bizarre! She is of the tech generation, the first to have email and internet and mobile phones. As I said barring actual learning difficulties she should absolutely be able to manage this just fine.
My mum is mid 70's and cracks on easy enough several of my friends are your mums age or older and they manage just fine.
Stop doing it for her she has to learn or else go back to older communication methods and deal with the difficulties of that in the modern age
If she can send a short text she can send a longer one.
Is she otherwise literate?
I have an uncle who is functionally illiterate but I suspect at least a mild literacy learning disorder there undx as he has difficulties with other cognitive tasks too and has a manual job he's had since age 14 he's also around same age as your mum. He is not lacking in intelligence he's very good at his job and is very knowledgeable on his hobbies and interests but he learned them by doing not reading/writing which makes a difference for him. He's pretty good in a pub quiz too as long as he's not writing the answers.
Have eyesight issues been ruled out?
I'm forever hunting for my specs and 9/10 they're on my head! I got fed up constantly losing them too so now I get supermarket cheapies so I've plenty of spares.
Your update at 1619 suggests to me there possibly IS an undx learning difficulty. It was not unheard of but relatively unusual to leave school with no qualifications in the 70's and has become increasingly less common.
That said I still think if she can manage a short message she can manage a longer one even if she sticks to monosyllabic words which is not a problem with texting.
I'm writing this on an iPhone 7 super easy to use (not my favourite phone for other reasons and I can't wait for my upgrade in a few months!)
I have friends and relatives who's literacy isn't great and they text/email/message me all the time of course, I'm a lit grad so I do notice the spag errors but as long as I can work out what they're meaning to say I don't comment, and even if that does happen (I can't understand it) I ask them to clarify in such a way as not to (hopefully) embarrass them.
We all have gaps in our knowledge - my maths is disgraceful! I'm ok on basic arithmetic but any "proper" maths even percentages and I'm lost! So there's been times when I've been asking them to help me work out something maths related. I'm also a bloody nightmare with anything practical, I cannot seem to get my hands to do as I'm intending and have literally hammered thumbs and caught fingers in hinges etc I'm a bloody liability to be honest, whereas the aforementioned uncle can do seemingly the most complex repairs in a jiffy.
What is she good at?