Sand & refinish is the only way to get rid of a discoloration in hardwood floors, and it's not fun if you've never done it before.
If it is deep, you can definitely bleach & stain. Floor companies will typically stain new wood to match existing wood, and you can usually get it pretty close depending on the floors. I've got Fir floors from 1880 in my house that are a deep brown/red with some crazy grain in spots, making it pretty hard to match, but the floor guys we had in last year did a pretty good job of fixing a bad patch (blah termites).
Anyway, the real problem is that you mention security deposit, which means she rents, which means the best course of action depends entirely on the amount of the deposit. If you or your friend have never refinished floors before, it's very difficult work, and hard to get right. If your friend does a bad job, not only is she out of the money it cost to get it done, but she STILL loses her deposit.