Location is Eastern Sierra with trailhead near Crowley Lake Elevations 8000 to 10,000 feet. This winter is mild so far. Lakes at 10,000' are often frozen in June. Normally June still has quite a few snow patches and crazy hungry Mosquitos. Bring head nets, DEET, and consider treating clothes with permethrin.
Expect overnight lows around freezing. Evenings and mornings may be quite cool. Especially as we age it becomes less comfortable to stay in bed until it warms up. I suggest a warm jacket and hat to allow your dad to enjoy the cool mornings.
Perfect your coffee making process before the trip. At your altitudes boiled "cowboy coffee" is great. (perfect temp for boiled coffee without bitterness)
EDIT: last time I was there cell phones didn't exist. I did drive down 395 a couple days ago though. I'm pretty sure you won't have cell reception at the trailhead or anywhere on your hike.