Well, you've not indicated what type of cozy mystery she might like and there's a number of sub-genres to that classification, including (among others) contemporary, paranormal/magic, and period/historical mysteries.
For contemporary stuff, the only person I can think to recommend is
M C Beaton and her
Agatha Raisin books and that's mostly because I'm familiar with her romance writing. Oddly enough (considering the
Murder She Wrote TV series supposedly inspired the creation of the term "cozy mystery"), I don't recommend the
Donald Bain Jessica Fletcher books because they never seemed to capture the essence of Jessica, though I understand they are quite popular.
However, I'm sure you'll get a ton of recommendations for contemporary and paranormal/magic series, so I'll focus on period stuff, which I myself prefer.
Now, on period mysteries, I
highly recommend
Carola Dunn's
Daisy Dalrymple series (currently 22 books), which is set in the 1920's in Britain. And if she doesn't mind a bit of sex and might like racier period mysteries, there's
Kerry Greenwood's
Phryne Fisher books (20 books), set in 1920's Australia. In both of those series, the heroine is in her 20s.
If you think your mother would prefer an older heroine, there's
Patricia Wentworth's classics with
Miss Silver (28 books, written between 1928-61). She's sort of like a mash-up of
Agatha Christie's
Miss Marple and
Hercule Poirot (also highly recommended - you can't beat the classics) since Silver is a retired governess/teacher who becomes a professional detective.
Some other period detectives that I enjoy:
- Rosemary Steven's Beau Brummel series (hero, 4 books, Regency period set around 1805)
- Bruce Alexander's Sir John Fielding books (hero, 11 books, mid-1700's)
- Deanna Raybourn's Lady Julia Gray (heroine, currently 10 stories, late Victorian Era around 1890)
- Tasha Alexander's Lady Emily novels (heroine, currently 10 books, late Victorian Era around 1890)
- Ashley Gardner's (aka Jennifer Ashley) Captain Lacey's Regency Mysteries (hero, currently 11 books, Regency period around 1810)
- Joan's Smith's Berkeley Brigade books (heroes, currently 11 books, Regency period around 1810)
- Andrea Penrose's (aka Andrea Pickens/Cara Elliott) Lady Arianna Hadley novels (heroine, 3 books, late Regency around 1814, all featuring chocolate recipes)
I hope these give you some food for thought and that you get a lot of recommendations for your mother.