Last time I checked on Coinbase, when you want to buy, it clearly states 'bitcoin,' 'litecoin,' and bitcoin cash' so I wonder how you ended up with bitcoin cash, unless you somehow thought they were the same and decided to buy the cheaper one.
I bet he think BTC&BCH are same coins, and as you say he decided to buy cheaper coin - from the viewpoint of inexperienced user it is much more desirable to buy
BTC at price of 400-500$ then 6000$. Unfortunately this is not an isolated case, internet is full of stories like this one - forked
BTC coins make great damage to original
BTC.