stage 3 stomach cancer need advice

My dad was diagnosed with stomach cancer last sept and he had 60% of his stomach removed last monday. He is recovering pretty good. The cancer was classified as stage 3, it did not spread to any other organs but the surgeon took 18 lymph nodes and 10 were infected out of the 18. He is scheduled for 6months of chemo/radiation therapy.

My questions, what foods will you suggest to help him put on some weight because i know chemo makes patients weak and lose weight. Also, what exactly does chemo/radiation therapy do to prevent recurrence? this is the biggest worry i have but i have all faith in GOD.

Does chemo target all infected lymph nodes or just choosing random lymph nodes in the stomach area? I am confused as to how it works and helps to prevent recurrence by killing the cancerous cells. Any advice will be of great help.

What is long term survival rate with such cancers? can anyone or long term survivors chime in and what tips to take to improve livelihood?

Also, he was diagnosed with h pylori back in 09 and was constantly treated for this with some meds by his doctor. He does not live in the USA so he visits for checkups. His last checkup was June last year and he complained of stomach pain to the doctor who just felt his stomach and insisted it was the same ulcer and gave him the same prescriptions as he always had. A month later, the pain got excruciating and my sister who is a doctor advised him to go for a colonoscopy/endoscopy and this is where they discovered it was a cancerous tumor almost blocking the small intestine. I feel like the doctor was negligent and will like to know if anyone had a similar scenario and what actions to take. lawsuit? or what?