Memoirs are great! Both my grandmother and great-aunt wrote memoirs when they were in their seventies, and I'm so glad we have their stories recorded.
But I will add a word of warning.
My grandmother's memoirs were well structured. Her recollections were set out in a logical order. Her prose wasn't perfect, but it flowed well.
By contrast, my great-aunt's memoirs were scattered and rambling, almost to the point of incoherence. Her stories jumped around. Her writing style was very convoluted. (Note, my great aunt had no issues with senility when she wrote her memoirs; she is simply not a very good writer).
As a result, guess whose memoirs get read more often, and discussed the most, in my family?
I think it would have been a kindness if someone had sat down with my great-aunt, and encouraged her to structure her memoirs a bit better, and write in a plainer style. Because she had some great and funny stories. But sadly, because they were poorly written, the impact was lost. If I were you, I'd consider doing the same thing for your MIL. Offer her your services. Tell her you love her memoirs, and that you'd love to sit down with her to "tidy things up" and make sure her little book is able to clearly reach out to future generations.