Why on earth are you so ready to criticize?
DH and I have done short-term trading on our own, with happy results - it pays our mortgage. Not day-trading. Perhaps SIL is not using the exact term, but it does not mean she is necessarily going to lose all her money! Especially if she is young and has time on her side, it's a good time to experiment if she is curious about trading.
Really. Some people are so judgmental.