Okay, I'd take highway 2 to Kalispell. You have to get through Montana somehow; it's just not going to be much different than ND. Glasgow is a cute town though.
Do Glacier while you're up in Kalispell; it's a must-see! Then take the Seely-Swan Valley route down to Helena via Hwy 83. You'll get breathtaking views and have the opportunity to see wildlife (just be careful not to hit any). This is a well-signed route; it's the second most popular route from Helena to Kalispell.
Stay in Helena for at least a day or two. The best thing to do here is to go on a guided boat tour of the Gates of the Mountains. It costs about $12 for adults and less for seniors, last I checked. You'll get spectacular views of the Missouri River and these staggering rock walls; a lot of it is surrounded by wilderness. You probably also have about a 50% chance of getting to see big horn sheep! The tour guides are great; you learn a little history, a little ecology, and a little geology.
The other good things to do around Helena are to go on the Last Chance Tour Train (great for seniors). It's a motorized choo-choo looking thing (open air) that you ride in. It's also less than $10. They take you around and show you all the old mansions in Helena and talk about Helena history. Very fun! You'll get some ghost stories, some state history, etc. It meets outside the historical society, so you should stop there while you're in town, too. They have great exhibits about Montana's Indian history and pioneer heritage. Also, you can visit the Holter Museum of Art for free! It's a great contemporary museum in the heart of Helena, right next to the spectacular cathedral. The Holter gift shop is a real treasure. Then, eat lunch or dinner at Benny's next door (they have jazz on the weekends). Take a tour of the walking mall if your senior can handle it, and visit all of Helena's downtown area shops. Stop at the Parrot! It's a Helena landmark. It's a family owned soda fountain where they make candy that's shipped all over the world. It's a step back in time. The Parrot is not tourist-y. It's just something that rarely exists anymore.
You can stay at Motel 6 or Super 8. I know that eating with a senior can mean needing curbside access or being a little less adventorous. So, the best places to eat in Helena are the Brewhouse (although more college & middle aged folks than seniors), On Broadway (you have to walk about half a block), Mediterranan Grill, and the Windbag. The Silver Star gets a lot of rave reviews, but it's just an overpriced any-city type of restaurant. If you really need to be parked close by the door and want pretty good, safe food with kind of a western feel, then you can eat at the Overland Express right next to the Super 8.
If you do a trip from Butte to Philipsburg, you'll get some great scenery. It's seriously a tiny, cute town kind of a hour or so away from anywhere else. But, it has a great feel to it and there a couple of options to go to sapphire mining establishments like the Sapphire Gallery. You sift through sand looking for sapphires (you do this outside). It's super fun and not something you get to do everyday. They also have a great old fashioned candy store, soda fountain, gift shops, etc. It's kind of like a trip back in time, and the surrounding scenery is great. There's non-chain places to stay, but you should splurge there and stay at the Broadway Hotel for about $100-$125. Here's a link to more info on Philipsburg:
http://philipsburgmt.com/
Make sure you click on some of the links from the home page to see some cute pics of this little town.
Another great place to visit in the SW Montana area is the Virginia City/Nevada City areas. You will be able to see exactly what Montana was like in the old west. As the web site says, it's truly a ghost town frozen in time (but it's still a real working town):
http://www.virginiacitychamber.com/. I loved this place as a kid! It's not a re-creation of a town; it's the actual former capital of the Montana territory.
I'd say the best time to visit is late June. Less chance of rain but not so hot that you'll be uncomfortable. There's usually also still some snow on the hills in June and the grass is really green. The rest of June is also great, but you have a greater chance of rain in the early part of the month. It really only rains here in May and early June with any regularity. Come when the grass is green! Also, it's a real concern that if you wait until late July or August, we'll be in forest fire season and we'll have much hazier days in western Montana (if not downright smoke!). By August, everything is brown.
Another tip, if you have a while to stay in Montana, plan a rodeo while you're here. It's really truly the event of the year for many small Montana towns. They have a good rodeo in East Helena the weekend after fourth of July. There's also a rodeo in Lincoln (a very scenic town near Helena) pretty early in the year--usually the weekend that falls before the Fourth of July.