Full sun. Water monthly or not at all in winter, start back slowly in spring until you're watering about once every 2 weeks (or 1 week if you live in like the desert). If you keep it outside, bring it in before it goes below 50°F. [This is what I do personally, others could differ.]
⬆⬆⬆ Savannah gave you good info. But I believe your professor's cactus is infested (the white stuff on the bottom) can't see it clearly so can't tell if it's mealy bugs or scale but it's most commonly affected by scale. Tell her to get neem oil or alcohol & scrub it with a Qtip... Meanwhile keep it out of sun.
Idk up close they dont look so much like pests as they do like a scar or something on the green part or water marks or something. Inwould still check for pests. I think they are the cinnamon or red bunny eats cant remember what they are called but also when I zoomed in i think they need water. They are a little wrinkly. Also.make sure they get alot of light so they stay nicely shaped and not stretched. Hers are a little stretched and let dry between waterings