1- Change vets. The vet's office should be able to do a fecal egg count in their office. Last time I got an FEC, I dropped off a fresh sample at 1pm, got called with the results at 4pm, and they're a small town vet, not some big fancy office with tons of equipment. If you can't find a vet that does it in their office, you might as well do a mail-order kit. The vet should also know which dewormers kill tapeworms; as mentioned, PowerPac doesn't.
2- Find out what the rotation schedule has been for the barn horses. Pinworms are supposed to be killed by every deworming product out there, so they are probably resistant to whatever the BM has been using (or she hasn't been worming like she claims she has) Zimectrin Gold or Quest Plus are good broad spectrum dewormers. This chart is from Quest Plus, but gives you a good idea of what kills what: