Earlier quoted context omitted.
I would not default to assuming it was his competitors, that sounds like scapegoating to deflect responsibility. What most likely happened is his site was scanned by one of the increasingly sophisticated exploit crawlers (anyone who runs an internet facing site and can view traffic knows what I'm talking about). His site got flagged as vulnerable, the hacker found out it was built like swiss cheese and had fun with i…
It's 100% this. Anyone who's run a website or web app for any length of time in recent years and makes a habit of inspecting their logs will quickly realise that they're being scanned by bots looking for vulnerabilities multiple, or even many, times per day. The search for vulnerabilities is entirely automated and will pick up any domain that has a website or web app attached to it. One those vulnerabilities are foun…
...and work to exploit your code to their own benefit. They don't do this just so that they can refund your customers.