[bluntus logicimus]

My first suggestion is to avoid the Banff townsite, unless they want to swap elbows with throngs of tourist shoppers.
Lake Louise is a definite day trip. As the post above suggests, the tea house at Lake Agnes is one of those "forever" memories - they will talk about it back home. I have seen many, many seniors take that hike up so you should at least suggest it to them. The hike has tremendous views of Lake Louise and the Chateau along the way so several places to stop and catch their breath. About 2/3rds the way up, you have Mirror Lake which is fed by the falls off of Lake Agnes. Again, another great spot to stop for a bit. All in all, the hike would be a decent day trip. If your friend's parents are coming in on the long weekend however, I would avoid the place. Absolute swarms of people.
Similarly, the Lake Louise walk is nice. Flat and scenic. You can rent canoes, if interested. Great patio at the Chateau. Pricey mojitos and well, everything is pricey.
As suggested above, Jasper is a rewarding trip. Edith Cavell is a great hike for seniors. Short, not too much of an incline and the payoff is Angel Glacier. A sub route will also take you to a large glacier pool which I am sure they would enjoy sitting around for a while. Athabasca Falls is another relaxed "seniors-friendly" hike.
Jasper would be a very long day trip however, so they would need to book in a couple of days for this one.
In between, as mentioned above, is the Columbia Icefields. Highly recommended but again, if trying this on the long weekend, you should definitely book ahead if possible.
Jasper and Banff are connected by the Icefield Parkway (highway 93) and even if they just want to stay in their car and go for a drive in the mountains, the Parkway is head-turning scenic the entire way.