Therapeutic levels of pot smoking don't necessarily cause impairment. A lot of people who get used to it can tolerate it better. I don't particularly like the smell and would never smoke it, but I've gotten to the point where I don't really see why it's so illegal.
I certainly wouldn't have any issue if I left my kid with my parents while they were consuming reasonable quantities of alcohol. If one or both of them were prescribed marijuana for a medical condition, I wouldn't have any problem if my kid was in their house.
But in OP's mom's situation, she lives in a non-legal state, and the marijuana is not prescribed by a Dr. so therefore a) she is doing something illegal, b) the amount is not controlled, regulated or overseen by a doctor for her particular medical condition, c) there's the risk that the marijuana could be laced with something, and d) the smell was strong enough that the 11 yo made comments about it, indicating that there was enough pot residual in the house for third-hand smoke exposure.
Now, I'm not in any way trying to invalidate the grandmother's medical condition or need for pain relief, nor am I trying to get into whether or not MEDICAL marijuana should be legal. That's not the issue here. The fact is that it's not legal in OP's state, so regardless of opinions on whether or not it should be medically, her usage of it is illegal. When you (general you) expose a child (and 11 is a really impressionable age) to something illegal, or perhaps more concisely in this case- to the knowledge that a family member is doing something illegal, what example is that setting for a child who is still learning about laws and responsibilities?
Secondly, when dealing with un-regulated marijuana (such as from an illegal source), you never know what can be in it. If a batch were laced with something, it could cause a variety of side effects that might not only be dangerous for Grandma (especially given she has a medical condition), but could be scary for an 11 yo to witness. Obviously, I hope nothing like that would happen and no one expects something like that to happen, but it is a chance I would not want to take risking exposing my kids too. Plus the fact that medical conditions really should be overseen by a Dr and with it not being a legal state, it's possible the Dr is unaware of the pot usage. This could have an impact on other medications interacting with it or just the body's response to it due to the medical condition, which can happen at any time given the changing nature of medical conditions, tolerances, and differing potencies and chemical compositions of street pot. Again, this could be a scary thing for a child to witness if something were to happen.
Thirdly, the exposure to the third-hand smoke from the pot residue within the home would be concerning for me in regards to my children. The residue left from smoke on surfaces interacts with common substances in the air and creates a mix of compounds commonly known to cause cancer and to be especially dangerous for children to breathe in.