There are more buses in the Luberon than before. I think bus service was expanded a bit due to the economy, but the focus is really on getting students to schools or local people to appointments in the bigger towns... not shuttling tourists around between the small villages.
I've done some research on this because we've had some people interested in staying in our Bonnieux apartment without a car. It can be done, but you're limited in where you can go and when. This is the website I've used for looking at bus schedules:
http://www.vaucluse.fr/2826-lignes-et-horaires-du-reseau-transvaucluse.htm. There's also this site where you can check out some options:
http://paca.tsi.cityway.fr/ri/?rub_code=4&noscript=0
I'm sure Kevin could give your friend info on what's possible using public transportation from St. Saturnin-les-Apt. (But just for example, I could not easily get from Bonnieux over there by bus... it would take several hours because I'd have to change and wait in Apt... but it's only 25 minutes to drive. I couldn't easily get from Bonnieux to Roussillon or Gordes either. I could go to Apt, Cavaillon, or Aix... and points on those routes, but only at a few times of day.)
The hiking is excellent and your friend could potentially walk to a few small villages within a few miles of Kevin's place... or there are bikes for rent as another alternative.
It's also important to understand where some of the bus stops actually are. Many of the bus stops have been recently fixed up, I think as part of economic stimulus programs. For example, there's a big new Bonnieux bus stop at the Pont Julien (on the D900), but you would have several miles to walk uphill to actually get to the village of Bonnieux. There's a shelter and places for people to leave a car at that stop if, say, they are going to Avignon for the day. We have two other bus stops up in Bonnieux and one also several miles away near the Gare de Bonnieux (where there is no longer a train station).
Local taxis would be another possibility, which have to be pre-booked. These are fairly expensive but could be booked for trips that aren't possible by bus or at other times. We do book our Bonnieux taxi on a regular basis to get people to/from the Avignon TGV station or the Marseille airport.
If your friend wants to really see a lot of the area and make the best use of their time, I'd definitely recommend a car.