Ideally all bettas would be kept in well planted ten gallons, but that is difficult to achieve. Personally my minimum for each fish is 2.5g, that could be by themselves or in a divided tank. However, with the right care, a betta can be kept in a 1g without problems. It's just more work for the owners.
Bettas can not get a bent spine from swimming in circles... As a rule of thumb, don't trust the employees. Usually they have the right intentions and sometimes they know their stuff but sometimes they give out ridiculous advice.