Nice!
Also, you probably saved them far more than $6000 - because the expense ratio is taken as a percentage every year, the fee over the rest of their investing lifetime should be enormous. For the same reason that compounding returns leads to exponential growth, the fees that mutual funds charge are compounding fees and grow exponentially over time (it just so happens that if you have a low expense ratio fund, this exponential growth takes a very long time to kick in).