@smack Agreed. I think the LW also needs to get him/herself some more salary information--right now s/he just has two data points: their own salary and one other person's. That's not enough to know if your salary is fair or not--maybe the other person is just a better negotiator than the LW. But if you have some solid data, like from glassdoor.com about industry-wide salary levels, that's something you can actually bring up with your boss. Also, if a company is known for underpaying people, maybe you really don't want to work there long term! There's a lot to be said for feeling like you are valued and fairly compensated.
No but wait, it doesn't matter only if you're not struggling financially right now and whether you can pay all your bills right now. You also have to think about how much you're able to save for retirement--that shit gets real scary when you start to get older, or at least when you start to think about getting older. You also have to consider that your future earnings are dependent on your current salary--your raises and staring salaries at new companies are based on what you earn now, so you are giving up a large chunk of income over your whole career if you don't get paid what you are worth.