It looks like it was caught early and she is likely to make a good recovery, just be prepared for a rough few months after the surgery and while she adjusts to her new stoma if she does end up needing one. 
she may need help with her personal care, changing her stoma and help with meals etc, she may be weak after surgery, sometimes the Bowel can go to sleep I think it's called an illius for a short while after surgery. If she does end up with a stoma she will have her own dedicated stoma nurse. 
My father had a few complications with his stoma his vowel became twisted and he had no stoma output, so please do look out for that he ended in back in hospital for two weeks. 
your partner may lose weight I think that's inevitable but she will put it back on once she starts to feel better. 
simple things like keeping her moral up will be so important in the coming weeks and after surgery, take her out in the car for short rides etc. 
help her to keep looking her best if she doesn't feel up to it by helping with personal care, this will make her feel much better. 
On the other hand she may breeze through surgery abs be absolutely fine, her surgeon and oncologist will tell you all about this at your hect visit and what can and can't happen my inbox is always open x

Charlotte, thank you so much for your advice! I'll definitely make sure to keep all those things in mind and try help in any way I can after the surgery. We saw the stoma nurse today who talked us through everything, I know it will still be a steep learning curve but she was excellent at giving us the first basic understanding of what it will entail. Thanks so much again, your advice has been a huge help and comfort.