I personally recommend 1, but you guys should decide:
Approach the Internal Committee, being a Big4 they'd probably take PoSHaW seriously. Proceedings have to be confidential and they'll handle it discreetly.
If she wants to confront him directly, she can call him out face to face. If he'a not adequately apologetic or fails to explain and remedy his unwelcome behaviour, she can fallback to Step 1.
I say "explain" cuz I, for example can't bear to look into someone's eyes while speaking to them, it always makes me wanna giggle... I'd look at the floor or at some point behind their head. However, I feel that such an explanation is unlikely, someone could definitely make out if you're staring at their cleavage. Moreover, sje needn't have to tolerate any behaviour that she finds unwelcome. The IC is the one-stop solution... overcoming any hestitation due to rank disparity also.