Moral dilemma

[Eavesdropper]

What would you do if you accidentally overheard a mangement discussion regarding a new appointment, and one of the candidates was discounted because she was a woman, who could potentially "disappear for a year on maternity leave". Illegal obviously, but would you do anything about it?

(I've changed my name for this, in case there's any chance of being identified by things I've posted in the past.)

Where I work, you don't need to "Overhear", comments like this, they are openly expressed by male collegues and managers alike.
I have one quite senior manager, who would think this a perfectly valid reason for not employing a woman. In fact I have a collegue, who was told to her face by him, that was the reason she did not get our head of department job. He also objected to the fact, that she had got P/T working, some years previously..... of couse these comments were made in his office, no witnesses, and only her word against his.

It there an HR dept you could mention it to?