If the tumor is one that is influenced by hormones then yes spaying will help. Ask your vet if the pathology says if it's that kind of tumor or if they can find out.
Years ago found same in my 6 year old bitch. Biopsy came back stage 3 and vet did surgery. He said "it went all the way to the stomach wall and I don't think I got it all." I was encouraged to do chemo. What I did was spay her ( because I felt that any more hormone activity might activate the cancer) and not do chemo. She lived quite happily and normally until she was 13 years old. Stay positive.
