not a gun dog problem but my sisters 20# terrier mix has worms in its poop. Can she use an over the counter de-wormer or is this in need of a visit to the Vet?

I'd take the pup to the vet.

Yes- to the vet visit

It is also very important, whether you treat the animal for worms using over-the-counter or take it to the vet that the area the dog does his business in is cleaned and sanitized very well or avoided altogether. Otherwise they will get worms over and over.

How old? if its an older dog over the counter should be fine just follow the instructions...

If you're seeing worms it is probably a tape worm and OTC wormer doesn't kill tapeworms. Vet will give a pill based on weight. One dose usually takes care of it. They get tapeworms from eating fleas so get the dog on flea and tick prevention to prevent further issue. Also, if this dog isn't on a wormer chances are its not on heartworm preventative either. A vet visit with a fecal and heartworm test is in order.