Learning sign language doesn't have to do anything with bad grammar in English, even if it was 30 years ago. Just the way the kids were taught in school system. Most kids, more than half in US, graduated high school with the reading level of a fourth grader or below. About 10% or less, the level of a seventh grader. As we all know, in order to be able to write, we need to start with reading.
Researches shows that if deaf kids were given the full access, which is their first [primary] language, ASL, and with a great support system both at school and home, I think they wouldn't have much of a struggle with English, their second language.
If you want to help your mom with grammar, you can start with a several basic rules such as nouns, pronouns, present, past, and future. -ed, -ing, etc. and subject-verb-object.