Good for you Toogie, for being ready to step up and help this person. Kudos too, to her, for asking, even though the two of you do not have a deep connection. She is in need, and without a doubt will appreciate what you can do to help make her life a little easier.
I haven't made one, but did help a friend plan one she stitched for another friend.
Your saddlebag concept sounds like a good plan.
Then buttons/velcro to secure it in place.
Getting her input will be important ...
* what is she wanting to put in the saddlebag?
* pouches and open pockets are good, and sized accordingly ... cell phone? portable phone? notepad? pen?
* zippered pocket(s) to keep some things a little more secure?
* inside use only? or needs to be weather worthy?
* does she travel with it? if it's going to be folded down, making the caddy so it can stay on, would be helpful
* likewise if it is a sitter walker, placement of pockets may be important for accessibility while sitting
* buttons/velcro/zippers etc. according to her physical abilities to access them
* walkers are different sizes/shapes/heights and different framing, so be sure to check the measurements with her actual walker
You can make it as plain as you want, or perhaps have some fun with it, using colours and fabrics to match her interests. Maybe even a quilt block or two? Quilting will help toughen it all up ... really you are just re-using your purse/bag sewing talents! Maybe a quilt block or two?
Have FUN with it!