Can a family member that was present during a pivotal doctor's appointment, but doesn't have medical POA share that info?

I've found myself in a tight spot. Grandma was being evaluated for dementia. She was having trouble with daily activities like turning on hoses, operating the dvd/vcr player she's had for decades, and learning to operate her new oven. Once accusing family of stealing her remote because the button was a different color than she remembered, and accusing me of stealing things like her garden tools which she later found in her hiding spot. This has been happening infrequently, until a headache last yr that lasted a week, for which painkillers had no effect. (Grandma has Von Willebrands{a bleeding disorder} but takes Excedrin like it's candy) She refused a doctor at that point. Then her behavior got aggressive, and she began yelling at me and my children for things that never happened. These moments went from happening once a month to every other day, and she started occasionally calling me by my mother's name, and forgetting my kids names even though she'd lived with them everyday for 2 yrs. She still swore she's fine, and continued to refuse a doctor. So I wrote down every instance, and presented it to my Mom who was in denial, until an argument over Grandma not being able to use her Costco membership card as a credit card. So then one thing led to another and we found her a geriatrician. She just turned 89, but is in good physical shape when it comes to muscles, bones etc. However, her doctor discovered her balance is horrible, her vitamin D was completely depleted, she's in stage 2 kidney failure, she likely has sleep apnea, she admitted to passing out in the yard or her room, and she scored a 20 on the MMSE test (parts of which he gave twice, but she failed to remember both times) Most of those are now accepted markers for dementia. However, since she's afraid of doctors, and this was just her first visit, she was scheduled to go back for a follow-up in regards to her memory. She has since been talked into turning on my mother and I (her caregivers) by her sons who only talk to her once or twice a week. She insists the doctor told her that she was in perfect health, just needed to take a vitamin, and was going to live to be 100 (what she's been telling her sons). What he actually told her is that he has some concerns, and that he believed she had 2.5 yrs if she didn't take his advice, and maybe 5 yrs if she did.{it's been almost a yr since then, and she has lost weight visibly, I'd guess about 15+lbs} She stopped taking her prescriptions consistently, then all together, and never returned to her appointment for a re-evaluation, or back to the doctor since. Mom was there the day this all happened, and witnessed everything, but Grandma refused that day to sign a consent form for information. Here's the kicker. Grandma has a loaded .22 pistol next to her bed, and has shown signs of paranoia in the past including a belief that if she didn't lock her car a homeless person would sleep in it.( we live very rural with no homeless people anywhere in 25+miles. And while locking your car is normal she insisted that was why she did it.) Since Mom was in the room during her doctor's appointment, and is a witness to all of this, can she share that information with her brothers? There are some confabulations with serious accusations now flying, and it will likely become a court battle to prove them false. It's almost as if she lost a huge chunk of her memory during the headache last year. I'm not sure disclosing her medical problems would even help, but it would be nice to know if there will be any trouble in doing so, and I'm really hoping to avoid a court case just so we can subpoena her records. I'm at a loss. I hope some of you have some experience that can help. (sorry for the novel folks)