If she has reasonable experience and common sense, she will be in less
danger alone in Yosemite backcountry than in most other places in
California, imho. I've solo hiked a lot in the Sierra and she sounds a lot
tougher than me.

But, May is still very early in the season in the Sierra. Most of the high
country will still be cold and subject to storm. For years I ran a field
trial in Carson City the last weekend in May and I probably got snowed on
about 1/2 the years going over the pass to get there. Late season storms
only last briefly, and the next day might be "hot" but without preparation,
it could be miserable. I'd suggest she stay at fairly low elevation and
certainly "expect" some nasty weather .... if it doesn't happen, so much the
better G