@BCS "Why doesn't he wear a ring then"
Not all cultures or religion believe in the ring as a status for marriage. The ring symbolizing marriage was from the Egyptian era, adopted by Greece and Rome, and from there disseminated to various parts of Europe. Since he's also an artist some of the chemicals in some of the paints could tarnish or damage the ring, in addition wearing a ring on certain jobs is very hazardous to the wearer (I've worked a few jobs where a coworker broke, lost, or had the finger reattached after an accident).
I think the fault is more along her line, she should have asked him if he was already in a relationship playing along the line he was. I assume most people in their mid 20s and older simply are, since he's a professor/instructor he most likely was from the sheer number of encounters he'd be exposed to relating to work (since most people are straight, and his job allows him to interact with both sexes, so playing the odds).