Very sorry to hear about this situation.
It is somewhat hard to answer this question without knowing some of the specifics of her situation. Some TIAs are caused by small
artery disease - which are very tricky to treat. Some are caused by larger artery plaques, which are often amenable to medical or interventional therapy. I try not to let someone's age deter me from intervening, but it might be that she is too frail to go through a procedure - the last thing you want for you mother is a complication of a procedure. Some are caused by heart disease, which should be treatable with blood thinners.
Another issue to consider is that these are not TIA's at all. I have found that small seizures, particularly in patients with very stereotyped episodes, can frequently mimic TIA in older patients. This is particularly true in patients with recurrent "TIAs" as "TIAs" should eventually cause strokes. It would be somewhat unusual for a person to simply have one TIA after another indefinitely. Older patients are at high risk of small seizures due to scarring and atrophy of the brain. An electroencephalogram (or EEG) could be very helpful to make sure that these events are not actually small seizures. Seizures are very treatable, and shouldn't be overlooked.