It's not clear from your question if you intend to (1) deceive your mother about what she's drinking or (2) will explain to her what Kombucha is and are merely trying to make it more palatable.
If (1) it's a bad idea. Many people experience gastric distress if they drink too much Kombucha when they're not used to it. Even 3-4 oz seem to upset some stomaches. Even she'll drink it down & keep it down, imho it's unethical to lie about what someone is putting into their body.
If (2) then using crushed cherries sounds like a great idea. I'd also add crushed fresh ginger root and definitely secondary-ferment for more than just a day or two. About 3-4 days should give it a nice fizz. It will be a tart drink but you don't want to overload a diabetic with too much added sugar. I have a sweet tooth myself but enjoy Kombucha like I'd enjoy a dry wine or a straight Scotch or Bourbon - it's not soda-pop sweet but it's not supposed to be. And limit her to 4 oz at first, maybe twice a day, until you know it's well-tolerated.
Sure you want to do this at a high-stress event (family gatherings at the holidays)? Our prayers are with you