I would talk to her first to assess her level of comfort with having you help her, having a bra fitter in a store, etc. I would steer clear of the department stores like Kohl's and Sears - they aren't going to have anyone really qualified to help. Even Victoria's Secret has a different market - and our local one has hired a college student I know whose entire background is in camp counseling and restaurant hostessing. She's working for minimum wage and I doubt she's had the training your friend would need! I would think on-line purchases would be nearly impossible.
Instead, try a place like Lady Grace which only does bras, and where there are usually qualified fitters for post-surgical patients. There's not only the fit, there's the issue of tenderness in the area, any reduced range of motion in the shoulder/underarm area (especially if lymph nodes were removed), and generally the amount of trauma to the surrounding tissues. Also, if there's been any radiation, the skin can be irritated. I've had great luck at Lady Grace but it's most helpful if you are comfortable being half undressed in front of a fitter. They are sensitive to modesty and it's helpful if you tell them what you are willing to do (e.g., be touched or have the bra adjusted, or just put it on yourself and have them come back it to check it for you, etc.). Sometimes by seeing where the first bra rides up or puckers, they can make an adjustment and suggest a better bra - but it's harder if someone is so uncomfortable that they can't allow this. Of course, women with surgical trauma and scars and who are just missing their breast are understandably more sensitive here.
She (or you) can also check with her breast cancer center - they often have a list of places to which they refer patients. I'm surprised they didn't already do this but maybe it's in the stack of papers she was given. In any case, a quick call can yield info about local resources.
Also have your friend wear clothes that are really easy to get in and out of, and which are typical of what she normally wears (so she can see how the fabric drapes over the bra, etc.). Don't make it too long a day. It can be emotionally exhausting as well as physically challenging.
If she finds what she likes, have her just buy 2, then live in them for a little while. Then, if she's happy, you can volunteer to pick up additional bras at whatever location gives you the best bargains.