Your lawyer is correct. Six month is a reasonable time. While there is no written law that say you have to wait for 6 months but if you come back within say 1 month then POE officer has valid reason to question your parents intent of immigration which contradicts the non immigrant visitor visa.
How do I know? Because I have seen it happened to one of my friends mom. She had to come back within 1 month due to family emergency here in US. The POE officer asked lot of question but finally she got lucky and let her through. If you do not have valid reason like emergency don't do that. It may jeopardize their future visitor visa. Just my 2 cents.