I'm sorry to hear that your fellow has been diagnosed with a cancerous growth in his groin and I understand that you are worried.
There are many types of cancer.
Some are very slow growing, don't tend to spread to other areas of the body and take a long time to invade into the normal tissues around them. One example of this is a hemangioma (a relatively benign tumor of the blood vessels). If he has one of those types of cancer then removal of the primary tumor can be curative.
There are other types of cancer that can be extremely aggressive. They spread tentacles of tumor cells deep into the tissues around the primary tumor and/or spread to local lymph nodes or even far away filter organs (like the liver, spleen or lungs). So removal of the primary tumor cannot hope to effect a cure, and we need to follow up with additional measures like chemotherapy, radiation or maybe even additional surgery.
Your veterinarian will submit the tumor for a pathologist to read and tell you a few things. One is what type of tumor he has (and thus whether it is likely to spread and where) as well as whether the margins around the tumor were clear of tumor cells. If the margins are clear it is likely the tumor was completely removed.
Your veterinarian can also do additional testing such as radiographs of his chest and abdomen to look for tumor spread as well as aspirates of the local lymph node to look for tumor spread.
The short answer is yes this may be a solitary tumor and removal could cure him. But we need much more information to know if this is true or not.
Please let me know if you have any further questions.