At your mom's age, and being "frail" and with a heart condition, consider carefully how the altitude in many western areas would affect her. Few places in Colorado are below 5,000 feet, and most roads through the mountains go over 9,000 feet, some as high as 11,000. On warm days it's not uncommon for the density altitude (how thin the air is compared to 60? air at sea level) is often near 9,000 feet at an actual elevation of 5,000 feet.
But if that's not a concern, she'll still need to minimize exertion when here -- remember it takes at least a couple of weeks to become accustomed to the altitude, even for folks without her age and frailty.
All that being said, there's a lot of gorgeous things to see here, even without much walking.
As to Yellowstone, and Wyoming for that matter, the altitude problem is similar. Do be aware that a very large percentage of the Yellowstone Park is out of reach for cell phones (no service), and it's a huge park. Traffic can cause slowdowns at times (I've seen it take over an hour to get a couple of miles in a "buffalo jam"). So don't count on getting to medical facilities quickly, should the need arise.
Are you going to tow a car with you? I ask because even the smaller RVs will have trouble finding a parking place at a lot of the sights in the Park, though some do have adequate parking for most class Cs, provided the cars haven't filled up the RV spaces, too.
Remember that in most western mountain (and often desert, too) areas, the thinner air and lower humidity allow it to cool off at night much more than tends to be the case in the east, and weather can change very quickly, so even a 90? high for the day might see the low at night down to 55? to 60?, perhaps an even greater change if a cold front comes through. In other words, be sure to have coats for (especially in higher areas) 35? to 40? on occasion.
Humidity is another thing to note, since it's not uncommon on a summer day to see humidity as low as 10%, which will dry the skin very quickly. As I age (I'm 74) I note many itchy areas that didn't use to be a problem, so be prepared for that, too. Speaking of dry, be sure that ALL of you drink lots of water when you're out here, even if you're not thirsty -- dehydration is a common problem in dry country, exacerbated by altitude, as well as the low humidity.
Hope this helps...