It sounds like she and I are about the same age, and she was into "Lite Rock" at the time I was diggin' the harder stuff. You might try some of the compilation CDs that are out there, some kind of Light rock, Mellow 70s, etc.
Greatest hits collections I would recommend of that type are by:
Barry Manilow
Simon & Garfunkel
The Carpenters
Seals & Crofts
Loggins & Messina
The Rascals - They're late 60s, but there is a lot of good stuff there, with the vocal stylings of Felix Cavaliere.
Perhaps a little Motown, like Aretha Franklin, the Supremes or the Temptations?
Also, Al Green...don't let the cover scare you off. I've got mine folded in the other way.
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I would steer you away from Jackson Browne (too brooding) and James Taylor, but JT only because on Steamroller, the last song on his Hits collection, he uses a 12-letter profanity that I doubt she would like to hear.
Doesn't like rock? Well, everybody likes Elvis.
Speaking of Manilow...he's got a couple of recent best sellers where he covers hits from the 50s and 60s. I haven't heard these yet, but I don't know how you could go wrong. His own stuff would be fine too.
You might check the reviews of any of these sets at one of the places they sell CDs online to see if you think she might like them.
As for newer artists, you can't go wrong with John Mayer. Can you imagine her listening to Norah Jones or Corinne Bailey Rae? I liked Norah's first one best (come away with me), and it picked up several Grammys.
Good luck! Let us know what you came up with.