the conversation with her doesn't have to be upsetting (and actually, if she does still have capacity for some decisions then the conversation should be with her). It can start off as a very general...'if you got really poorly would you want to go into hospital or would you prefer that the GP treated you at home' and take it from there. If there's no way on earth she'd want to go to hospital then that answers the question - CPR would be utterly futile without the full support of intensive care. This is how I started the conversation with my cognitively impaired granny, and when she caught us a bit by surprise a fortnight later, it was a huge relief to know that she wasn't going to be put through a completely futile resuscitation attempt and she died peacefully in bed. (to start the conversation I think invented a friend who's parent had become really sick and needed ICU and I think I said something like 'not sure I'd want all that for myself Granny, would do you think?...').
It may be worth asking the GP just how healthy she really is and perhaps if there's anything they can see now that's prompted them to raise the question at this point - it's not uncommon to not have a long list of diagnoses on paper, but to actually have knackered kidneys, evidence of heart disease, etc (and she clearly has a neurodegenerative condition). Is there anything else in the background that just hasn't been given a label yet (dementia is often a small vessel disease, and as well as there being small vessels in the brain that can become damaged, they're in all the other body systems too). As part of the normal ageing process we lose reserve, so even a perfectly 'healthy' octogenarian, for example, will be using every single nephron in their kidneys, myocyte in their heart, brain cell, etc. They function absolutely fine until they break their hip/get pneumonia and then the wheels fall off in some style. An average 50 or 60 year old will still have 'spare' cells they can lose to a major illness/insult before it becomes a big problem.
If someone has a very sudden cardiac arrest and receives immediate good quality CPR (from friends/family) and gets defibrillated quickly then their chance of surviving will often be relatively high. They will have accumulated some brain damage and other organ-system damage during the 'downtime' though even with good CPR, but if they have enough reserve that might not be a huge problem. They will require a stay in hospital (which is disorientating for patients with dementia at the best of times) and they will require invasive management which can be pretty unpleasant but it may be possible to recover to a reasonable quality of life though and is the right decision for some people.
If a cardiac arrest is the end result of 'whole body failure' (eg. due to an infection) then that is more likely to be unrecoverable and is a much bigger ordeal if they do survive it. Getting a pulse back is only the beginning and recovering to hospital discharge may be a much bigger effort for them. A more common outcome in this scenario is that they will survive the initial arrest but become adrenaline dependent. Every time the adrenaline wears off the heart stops again, but carrying on the adrenaline causes more damage to the other organs.
A last thought (that applies to all-comers), is that if someone does have a cardiac arrest the very best chance of survival comes after receiving bystander CPR (+/- defibrillation) before paramedics arrive. So if you/she/your siblings do agree that resuscitation is the right thing for her, then it's definitely worth whoever she lives with having done a basic life support course so they can confidently do CPR to give her the best chance of surviving reasonably in-tact (that really applies to everyone, not just people with older relatives).
Resuscitation isn't always the wrong thing for older folk, but it is absolutely nothing like what you see on the telly and it's really important for people to understand the implications of that decision, what the likely course would be if someone survives the initial insult, etc. It's probably worth going back to the GP once you/your mum/your siblings have spoken to each other and maybe finding out a bit more about what their thoughts are too (and possibly asking about a ReSPECT form or whatever local equivalent they use... which documents preferences about medical care including, but not limited to CPR).