Sorry to hear about your wife's brush with breast cancer. Hope everything is clear and stays that way.
As a turner and physician (anesthesiologist), I can certainly weigh in with an expert's opinion regarding the lymphedema aspect. The turning part, I'll leave to my capable colleagues, some of who already chimed in.
Lymphedema, as you likely know, is one of the annoying side effects of surgery involving removal of some or all of the axillary lymph nodes. The stuff that normally leaks out of your blood vessels (lymph), and is usually returned to the central circulation by muscle activity and the lymphatic system, is stymied by the lack (due to surgical removal) of the collecting pots (lymph nodes). The arm can swell and become painful. In some situations, infections can be very serious, as normal lymphatic activity assists the body in removing wastes and fighting infection. Okay, enough school.
I would think that, if she has no lymphedema at this time, I would not worry about it. She either will, or will not (brilliant ey?). Might as well wait until it starts before loosing sleep over it.
Physical activity, like wood turning, especially using the upper extremities above the heart, should promote better lymph flow; even with a deficit. Also, having a hobby that you enjoy is good for overall well being and particularly helpful in combating disease. It should not hurt though. I didn't notice if you mentioned how long ago the surgery was, but it can be several months before her upper body gets back to baseline. She's gonna' be sore for awhile. Physical exertion is good, but I wouldn't push it into the pain. All in all though, wood turning, in and of itself should be safe. Be careful though about splinters and cuts on the affected arm. As I said, infection risk is higher for her (at least in the short term) than the average person.
Perhaps some different tools (carbides? captured tool rest?) or even a swiveling headstock lathe (if you can afford to do that) might make life easier for her. Ask her surgeon. Support her (as clearly you already are doing). Encourage her. Time will tell whether or not any of the less fun side effects of the surgery will rear their ugly heads.