He didn't, he was on a sick day. Dishonest about being sick, but still not on company time. And more and more companies just lump sick time and vacation into one pool of "paid time off" so it doesn't really matter what you call it.

Edit: in fact, we don't even know that his company pays him at all if he's on a sick day. Many jobs don't, e.g. if you are paid hourly.

Exactly. Whether you work on your projects in the evening or during vacation/sick days is the same thing. I never provided reasons for my sick days and it wasn't requested/required, so I didn't lie either. I feel that having people justify their time off worsens a company's culture.

This.