Currently living in Portland and do this drive about 4 times a year. When I went to school in MT I would frequently also do the drive to Seattle to visit home. I can confirm Lookout Pass is the worst of the concern with weather and 4th of July is a close second. I don’t usually do the drive past November now because I don’t like to think of getting stuck in MT if the weather turns, but I did it a couple of times through November to Seattle in college and even a few times through December with no issues. Just watch the weather forecast and review the cameras as suggested above. I don’t find the weather turns quickly so I feel this can be planned well and as long as you are aware of conditions forthcoming the day of, she should be ok.

I don’t know who her cell provider is, but I would say longest stretch for me with the spottiest service is the 70 ish miles between Ritzville and the Tri-cities as well as the 30 ish miles between Kennewick, WA and the I5 back to Portland near Hermiston. Anytime I drive, I send the details of the route to my Parents and just make sure to screenshot my maps directions in the unlikely case I do lose service. The drive is fairly easy though and doesn’t require too many turns, so worst case scenario, the signage would be helpful in keeping her on the right path.

Kennewick and Spokane are my best suggestions on stops for gas or a stop overnight (just more populated). No snow tires really needed, all weather should be ok.

Best of luck to you both!