I've used Manjaro with both KDE and XFCE. Both are quite good. Rolling release may not be the best choice for the non tech savvy. For a non rolling release I like Mint with Cinnamon.
I would personally go with Ubuntu 16.04 Mate. Rolling releases are generally not something to give to beginners but if you really want a rolling release I would like many others have suggested go with Manjaro.
OP, what does your mother need the machine to do?
already solved dude. She likes the themed Ubuntu Mate 16.04 desktop I set up. done.
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it resembles windows in coloring and style. She likes Linux and has used it on her laptop since 2008.