Should a dog be put to sleep just because it suddenly started having seizures?

Our dog is around 16 years old. She does have a tumor on her back leg. She had a heat stroke few weeks ago accompanied by a seizure. She was ok for a while, then she had a seizure today and yesterday. It's still warm out, but not hot as it was. Anyway, my dad said he thinks before winter, she needs to just be put to sleep because she's suffering. I told him not to do that. Seizures aren't necessarily painful.