I'd be absolutely honest with her (not maybe that your dog is frightened of her/her van) but that your not happy with the volumn of dogs she walks at once, her transport situation or the insurance issue. It may well prompt her to change things for the better.

My biggest concern would be not being insured.

If she's doing it as a business she should have business insurance.
I used to use K9 Capers dog walking service and they came armed with police check records, insurance, prober forms to sign that you give them consent to take your dogs etc and they were great.

The girl in my area gave it up after a couple of years, so on searching for another walker I got a guy who does it as a self employed business and he too has business insurance.

Neither of the above walkers took them in a car, they would walk locally and NOT with a load of other dogs, this giving prober attention and care to them!

I also made a point of being around or bumping into them on ocassion to see how the dogs were with them and it gave me piece of mind that they were very happy and would go running up to them for a fuss if they saw them around.

hope that helps, Victoria