In fact the "ideal" scenario is that two employees interact, one rejects/rebuffs/corrects the other for harassing or inappropriate remarks and it does not continue. IF it continues you have something to work with, at present it does not sound like you do. Thank your employee for telling you and for speaking up, and ask her to keep you updated.
Sexual harassment, like other workplace harassment issues, generally has to be considered "pervasive and severe" to be something with teeth, and yes the court has generally very high standards for what constitutes both of those things.
Talk to your HR rep and do not burn up a ton of cash on lawyers just yet. Talk to them about this as a constellation of fit issues for the role, see if term is needed for all of the above. Make plans to hold a sexual harassment and bullying training anyway for all EEs. Let HR help you with this, seriously, we deal with it all the time.