Andrew, if she isn't a believer then most of the puritan or reformed devotionals might be a bit daunting. Reformed people are more theological savvy than the average reader. Most of the puritan tomes tend to be wordy. Here's my suggestion.
I gave my mom My Utmost for His Highest. MUFHH is very readable and yet dense enough to stimulate further thought and meditation. I still like this devotional.
Chambers was very conservative and reformed (small 'r') in his early years. Later on he tended toward the emotionalism of the charismatic movement but there are only slight hints of that in his devotional. You can get the original or modern language version.
My other suggestion would be Daily Light on the Daily Path. This devotional is just scripture verses edited together into a specific theme for each day. There is no commentary, just verses. Because the verses are edited together at someone's whim one might notice a bit of commentary intrinsic in the way the verses have been combined, but otherwise it's pretty safe.
Of course I would second Dan's suggestion for Spurgeon.