Tough situation you're faced with. To answer your questions - If I had to choose between the before OR after surgery, I think I would say after. That was when I needed the most support from my wife. (If at all possible, try to also get down there before also and help get her things in order. But again, if I had to choose, I would choose after surgery.)

You can help her get a support system in place. Rides to and from the hospital for the surgery and possibly chemo or radiation treatments. Coordinate with the hospital and verify all insurance issues have been addressed. Find out where she keep all of her important papers that you may need to access. If she ends up using the PEG tube and pump for nutrition, make sure she understands how important her feeds are to her recovery and long term health.

There are so many things that can be talked about in great detail.........But I think the most likely feeling your mother will have is the frustration from losing her ability to verbally communicate. If she doesn't have a smart phone, or a I-pad, buy one for her. Texting and email may be her only means of communication with your or other family members. The fight to rid herself of cancer will be her focus in the near future, but there is also a psychological / emotional adjustment to be dealt with long term. The latter is sometimes a tougher one to break trough.