Well, there are multiple options here, but off the top of my head I'd probably use an hp threshold mechanic.
Basically, characters below X hp turn to stone if they see her. Characters at or above X hp are lucky enough to sense something is amiss and only catch a glimpse from the corner of their eye, resulting in being restrained rather than petrified, as their body stiffens.
That removes the gotcha factor (unless a PC didn't heal after taking damage, which is itself a choice). But it's very unlikely that half the party will be petrified in the first round, unless they're pressing forward when they should be retreating.