I don't know if the original poster has checked back in on this discussion, but I would love an update on your progress with the adult you are helping. Please let us know if any of these suggestions have been helpful to your student. Thanks!
I totally agree with clairewait. Nothing will improve a person's spelling and vocabulary skills than constant reading. Visualizing words in print and how they act to form sentences will help your student better than most educational tools.
I would agree with #2 that lots of reading will be a big help with your student's spelling. Moreover, having them write without the use of spellchecker would be a big help. When using a word processor with spell check and auto correct features it doesn't make people think about spelling. Have your student write, then correct the spelling. Practice the errors that come up, and hopefully you can start to trim down the number of random words causing issues.
I hate to be the first answerer when my advice is possibly not going to be exactly what you are looking for. I find that teaching spelling--specifically--especially for older student and adults, today, is a bit of a waste of time.
My advice is to encourage this student to read, read, read. Also, incorporate the use of a word processor in her work. I do not consider ABC check to be a crutch, but rather a pretty useful tool for most adults (esp. in the professional world) to rely on for improved spelling.