I suggest you send your friend some links to the following places, which would be nice in May, and ask your friend to prioritize them:
Grand Canyon
Yosemite
Hawaii: Big Island (w/ Hawaii Volcanoes National Park)

I live in San Diego and I don't like Los Angeles, but you might also send your friend some links to some of the city highlights and see what your friend prefers to do.

I would skip Las Vegas unless you need it as an overnight enroute to Grand Canyon.

The Grand Canyon will be fine at that time in May. Same in Zion and Bryce or Monument Valley. Vegas will be hot but not as bad as June or later. Phoenix will be the hottest place but you can drive out of there to higher elevations in a very short period of time.

Yes, by definition it should not be too hot when visiting a canyon, as long as we are on its top. But we have to move between them, and the only point to fly in is Las Vegas. I remember being on a couple of business trips to Palm Springs in the summer, and it was beyond unbearable.