Could you tell me what happens if a brain cancer patient goes into a coma?

Depends on cause: This is certainly concerning particularly if it is related to the brain cancer. After the other non-cancer related causes have been ruled out (ie. Drug reaction, hypoglycemia, hypoxia, seizures), then you need to worry about if there is exacerbation of the brain cancer. That is, is there increased size, swelling, bleeding that could adversely affect the brain function and thus lead to coma.