He was on a visa waiver - if he is British that allows him to do anything you can do on a B1 business visa. In fact the only reason you have to get a B1 visa is because you can't use the visa waiver if you are, say a convicted criminal!
You can work for your outside-the-US employer, you can collaborate with US customers, suppliers, partners, you can train them,you can sell, demonstrate and install products. About the only thing you can't do is work for a US company.
Picture if he was an engineer from Rolls Royce aero engines going to Boeing - everything that engineer might need to do you can do on a visa waiver.
Indeed, but if you're self-employed, then you should either say you're coming from (your own) Foo Corp. in Australia to have business meetings with Bar Corp (in US); or follow GP's advice and say you're on vacation.
Despite your visa waiver, you still have customs forms to fill out and they do ask whether or not you are on business or not.
They're very clear that having a waiver, or even a visa, is not a guarantee of entry into the US and that ultimately it's up to the CBP agents.
Even if you just intend to visit and and up doing some work you'll get in deep trouble, but if you lie about it from the start it'll be much worse.
And they do sometimes check.
A tourist visa and a work visa are not interchangeable.