Hydrometers are of little use because of the extra acids and other bits that throw off the reading. You'd need a conversion to compensate for this, in addition to the lack of ethanol.
In my experience, no yeast needs to be added to secondary. There is more than enough left over from primary. If you use raw fruit with the peel still on, they'd be covered in yeasts anyway, so you'd get extra from them.
Your buddy probably wants you to figure it out so they can benefit from your research. Tasting the brew is more helpful than gravity readings. I used to record gravity readings and pH, but they were all over the place every time I brewed, it seemed random, and very unhelpful.
For the amount of fruit/gallon, this is mostly a personal preference. In my apple spice recipe I use 1 whole green apple, 1 whole cinnamon stick and 3 whole cloves per 2-liter. You may like less apple and more cloves. As with most things, start with modest amounts, take notes, and adjust on subsequent brews. Typically, a handful of whatever fruit you like is good for 2 liters of KT. Herbal additions are small to large pinch or a couple grams.
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