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Someone just told me that he was taught that a man should not walk by a women's mikvah at night

Is this true?

Is there a source for this?

DanF's answer is essentially true; a woman's Mikvah night is supposed to be as private as possible, and men walking by makes it less-private (as well as uncomfortable for the women involved). I'm just going to add a source that says explicitly that going to the Mikvah is supposed to be done in a discreet manner, Kitzur Shulchan Aruch 162:10:

יש לאשה להיות צנועה בטבילתה להסתיר ליל טבילתה, ולא תלך בפני הבריות שלא
ירגישו בה בני אדם, ומי שאינה עושה כן נאמר עליה ארור שוכב עם בהמה

A woman should be modest about her immersion, and conceal the date of
her immersion. She should not go to the (mikvah) when people are
about. [She should arrange it] so that people will not notice her.
Concerning a woman who does not act modestly, it is said, "Cursed be
he who lies with any kind of animal."

While one can argue that this is an obligation on the woman and not the man, it definitely shows that it's appropriate for men to not congregate around Mikvahs and make things difficult and uncomfortable for women.

It is definitely true! However, there is no direct source (i.e., one that specifically says, "A man should not walk by a women's Mikvah.") This is just a derivation of modesty and privacy. A woman attending the mikvah is meant to be absolutely private. If a man walks by a mikvah, he may recognize some woman going or leaving the place, and now, he knows private information about her menstrual / niddah schedule.

I can personally vouch for this rule from what I recall when my wife went to the mikvah. Besides, a huge sign in front of the door requesting that men not pass the mikvah at night (I noticed this sign during the day, BTW,) they also requested that men drop off and pick up their wives around the corner from the mikvah. They didn't want you waiting in your car by the mikvah.