Longer and more frequent visits will put her on CBP's radar.......and if they even suspect she is abusing her B visa, they can immediately revoke it, and send her home......she could even be banned for 5 years. Short infrequent visits is the safer route.
On another note, staying 6 months is dangerous because any delay and overstay will immediately and automatically void her B2.
Common Rule of Thumb: If she stays 6 months, then she should remain outside the US for a 6 months to a year before re-entering.