"Is she on the heavy side?" No, thin, petite.
"Does she work out?" No! If she does it would be very little!

If she's small, little fat on her, and little, if any, exercise, then it could very well be the anxiety setting off the sweating and feeling cold at extremities furthest away from the heart. Anxiety sets off a chain reaction, stressful fight or flight response. Part of the response is skin pores open for sweating, stress hormones are released, blood vessels dilate, organs slow down, and blood goes to muscles. Some people feel short of breath, palpitations, and fear afterwards.

There's always some trigger like an event, object, situation, thought, or some other stimuli like food. Other's here have pointed out other potentials, but it's hard to say what it truly could be. I like to try simple things first like exercise (she needs to sweat), change of diet, relaxation, and identification of stressors. I also recommend a food-based multivitamin with active food-based B-vitamins.

A simple resistance training for a few minutes a day will help. For example, exercise bike for  5-10 minutes non-stop or until sweating. It will help drain the adrenaline and improve blood flow.

Increase foods that reduce stress response like green veggies, nuts, cheeses, yogurt, berries, and foods like in Tryptophan. Avoid stimulants for a while like any caffeine.

She could take some time out in the middle of the day and have NO stimulation. For example, a middle day nap. Or listen to music for 20 minutes with eyes closed. Read a book. Something stimulating the mind to slow down.

Some foods can trigger anxiety immediately, or delayed, sometimes over a whole day. It's difficult to do elimination diets for younger people, they think they're invincible often. That's why improving nutrition and actually exercising will help most cases, as well as avoiding stressors or triggers.

"Does she sweat all over or just the hands? Or an isolated part of the body?" Hands, feet & underarms

"Does she take any medicines?" No

"Does she have any other symptoms?" No, not so far as I know.

"Hyperthyroidism could be a cause of sweating, does she have any other hyper-T symptoms?" I've asked a few questions, and so far no...will find a list of symptoms and ask her more questions.

"Cold hands after sweating could be a side-effect of releasing heat through sweat" She did say her hands are cold regardless of sweating.

Some more info is found here (they don't push drugs): http://www.womentowomen.com/depressionanxietyandmood/anxiety.aspx