A stock will only be hard to borrow for shorting not for buying. And if it's to really short a hard-to-borrow stock, it will be to buy a put and then exercise it. It wouldn't be to buy a call. And usually when the stock is really hard to borrow, the broker wouldn't even allow you to keep the short transaction open; it will demand you to buy back the stock to close the short right away so it'll be a much ado about nothing.
Or bought a call instead of a put and then exercised it thinking it will protect against a price drop. Either way the guy is a noob.
Actually it was a slightly cynical response from me (sorry!). Because you start with a completely wrong stock price (as parts of the day of exercise the option was considerable ITM) and put up a whole story after that. But anyway, we leave it to the (lucky!) TS to filter out the useful parts from this topic...
The strike is at $18 but the market price of the underlying is at $15 and somehow the OP got assigned. That's how I understood it. Thanks for clarifying your sarcasm.
No, underlier was around $19 that day! (he even says "name at $18.70" but that is not relevant as exercise was done the previous day)
The few times I have had early exercises that did not make mathematical sense have all occurred in periods of market sell off, I think there is some people who lose it during these times.