One problem is that she insists on doing things in her head that she should NOT do in her head. Another is that she squeezes everything into tiny spaces to "save paper" and doesn't write her work down in a clear, organized, and sequential fashion. Finally, and to me the most concerningly, she also says that she is somewhat crunched for time. Thoughts?
I had very similar troubles when I was in school. Mental math was my strong suit and I felt slowed down by the task of writing it out. I always had mistakes and not because I didn't understand the math.
My dad was a math teacher (not mine) and he applauded the use of mental math. He gave me a couple tips on showing my work.
1. If I solved it in my head, write down and solve the inverse of the problem to check for errors. It's better than showing nothing.
2. Take extra paper (if allowed!) and solve one problem per page. Seems like a huge waste but it gave me a clear "mental space" to deal with one problem at a time. Turn in all of the working sheets with the test. Stapled. Because every math teacher lost one or two of my extra sheets.
3. Solve the hardest problem first, and work to the easiest. Often math tests have the easiest problems at the beginning. This is a time use strategy for feeling less stressed out about the amount of work left. If problems 1-5 are easily solved in my head, but problem 6 requires a huge amount of effort, then once the hard one is complete I won't feel as rushed to show my work for 1-5.
I hope that helps! It really helped me. Now 20+ years later I still do mental math every day.
