>In general, women tend to negotiate less aggressively (there's research supporting that), so if your friend was paid less, it doesn't necessarily mean there was discrimination.
The real question is why aggressive negotiation for doing the same job should lead to fairly different salaries. Forget gender, race, etc. If I can negotiate a higher salary than my coworker who does the same job, I'm not seeing a really good reason why I should get paid more.
If we are going to normalize this reason, then it's two faced for a company not to mention it. When it comes to the internal dialogue, performance reviews, etc, are all about the work you did, and how it impacted the company. If aggressive negotiation is a valid strategy, I would like the company to be transparent about it. List it at the same level as your work performance when listing the criteria by which you are evaluated.
Edit:
Too many of us treat salaries as a game. When we play board games, we often accept rather arbitrary rules. One of the rules that many put on this salary game is: "Well, he gets more pay for the same work because he can negotiate well". Somehow, we don't accept it as a rule in the "school game": "Well, Alice got a higher grade than Bob on this essay because she negotiated better. She brings me two apples a week, and recently threatened to stop giving me apples and give them to Ms Brown of geography."
I'm not, BTW, arguing against negotiating if you're an employee. I'm arguing that companies paying significantly different for similar work just because of negotiation prowess is fundamentally an admission that "Yes, we will pay people differently for the same work." It shouldn't need to become an issue where one group (women, here) feels disadvantaged. It's a problem even if there is no systemic bias. The appropriate thing for the company to do when an employee shows that the market is paying better is to do a salary adjustment (if they can afford it) for all similar employees (barring small deltas for individual performance differences). Without this attitude, they are fundamentally creating an "us vs them" scenario ("company" vs "employees"). Sooner or later, that mentality leads to a large amount of cynicism and an unwillingness to view themselves as a cohesive group ("I'm just going to do my job - I don't care about corporate goals, and am not here to improve the company beyond what they pay me for").