2. Live below your means so you can save ~50%
3. Continue for six years
4. Boom, you're up 300k from where you started.
There some minor things you can do to boost your savings like make sure to put all savings in low cost mutual funds and don't save a penny until all loans are paid off (and pay them off in order of interest rate, not in order of magnitude of the loan!) but that's the essence of it.
100k a year will probably end up being ~65k after taxes, which means living on 32.5k a year to save 50%. For a frugal single person that's doable, beyond that it gets tricky (if you're in the kind of city where 100k and above salaries are common, you're probably paying 30k a year in rent alone).
And even then, that only nets you $195k, not $300k.
Sales tax won't substantially change things because housing is usually the biggest expense and isn't taxed. And if you're saving and being frugal you're not buying many taxable items.
People tend to overestimate their tax burden. The difference between your gut figure and the actual numbers is almost 10% of income, which is a huge difference.
I think it's great that she paid it down, but on 225+k/year, it seems like the interesting story would be if she didn't pay it down.
I make great money myself and am married, and I don't even eat out that much.