Glad you had a nice day together, it is better to laugh and be as normal as possible.
Ask to go with her for the prosthetic fitting, I went with my mum and she said afterwards she was glad she wasn't alone.
My mum then went with my grandmother, her MIL who she had a fraught relationship with to say the least, for her fitting.
Wigs vary in price, so do the products to care for them so do some research on them if she opts to buy. Areas with large numbers of orthodox jews often have good wig supplies and hairdressers who can style them.
Have a sweepstake on what colour her hair will grow back, my mum's mousy with lots of grey grew back black, then the second time it has come back a uniform dark grey.
My mum has had great support both from the local hospice and a 'group' which I think the hospice organised but was all women with/recovering from BC.
The hospice ran a 10 week 'course', basically you went for a day, took part in some arts / crafts activities or some physical such as tai chi, had a nice meal with a glass of wine and then a talk from various groups, their favorite seemed to be the guide dog, well the dog didn't talk but the owner did and told them about training and various tales of his dogs.
This was for any woman with breast cancer to attend.
Another thing was organised through a charity was housework and in particular ironing. There might be something similar or maybe you could help out.
With her sons I think if you could take them one day a week for a meal and play in the PS whatever / xbox for an hour of two. It would be good for them to have an adult to talk to who is not their mum. They will be scared too.
You don't mention if your friend has a dh / dp, apparently I was the only person who asked my dad how he was coping.