If she can have gelatin, you might try reserving some gelatin when you pour it into the container to heat it - I use a pint Mason jar - and sprinkle a packet of plain gelatin on top, let it dissolve while the milk heats. When the milk is hot, I add so of the hot to the cold, cover, and shake it to make sure it's dissolved, and then I add it back into the milk. Helps to thicken it a bit.
Also, I find my yogurt is frustratingly watery in the Winter; Summer temperatures make for a thicker smoother yogurt. I've started using a heating pad under the jars on the countertop to help keep it warm, in addition to all the towels I already pile on top of the culturing milk. LOL How long are you leaving it to culture, and are you able to keep it warm long enough?
And if all else fails, I have been known to strain off some of the liquidy whey (you can use it as a starter for your next batch) to thicken the remaining stuff.
You might want to try adding the slightest bit of sugar to the mix to feed the probiotic bacteria; maybe add a spoonful of sugar and dissolve it and see if it makes a difference?
