Go to your favorite purveyor of fine music, find “Passion: Music for The Last Temptation of Christ” by Peter Gabriel and let nothing stay your hand. This album covers all your bases.
Enya might be a bit TOO soporific, but she otherwise fits the bill. Very soothing. Give her best of, “Paint the Sky With Stars” a spin. Her sister is the lead singer of a similar sounding band, Clannad. Also in the “vaguely celtic” archipeligo is Loreena McKennit, whose release “The Book of Secrets” is probably a good fit.
The Dead Can Dance Ouevre might also prove rewarding. Their best-of is “A Passage in Time”.
The Cocteau Twins “Heaven or Los Vegas” is tuneful, fairly light, and upbeat, thought the singer is (purposefully) unintelligible.
How closely does she listen to lyrics? The Beautiful South have release a half-dozen+ albums of mid-tempo, pleasant sounding rock music. The caveat being that they have two basic kinds of lyrics: heart-felt paeans to the glories of growing old (When I’m 84, Perfect 10, 'Til You Can’t Tuck it In) and incredibly bitter songs of betrayal, lust, and despair (Old Red Eyes is Back, Woman in the Wall, Mini-Correct). But the music is always lovely, bright, and upbeat no matter what the lyrics.
I can’t go through a music-recomendation posting without mentioning my favorite band, XTC. For what you’re looking for, try “Apply Venus Volume 1”, “Mummer” (except for the last song, which is a real knee’s up), “Nonsuch”, and particularly “Skylaring”, a downright beautiful album.
Howabout the Beatles? Is “Sgt. Peppers” too frenetic?
For more recommendations, what’s already in the collection would be useful info, along with what bands you know she already likes or dislikes.
Also, I think most of the bands I’ve mentioned above should be available at your local record store, regardless of size. You may want to download a track or three and run 'em past your mum for a thumb’s up or down before investing.