I have fine wavy hair but when I used a flat iron regularly, I ended up with broken hairs all over my head that stuck straight up and looked worse than frizz, so I flat ironed more to get those down and more broke and it was a vicious cycle. If she HAS to flat iron, make sure you use a ceramic
flat iron and apply lots of some type of heat protectant first. This shouldn't be a daily occurrence either or else my guess is she will see bad effects within 1-2 months, at most. If you could encourage her to embrace the curls, she will be happier in the long run, with healthier hair and not having to fight the weather constantly (anytime it rains, snows, the wind blows the wrong way, that flat-ironed hair will frizz and curl). I am still growing out some of those little broken hairs.
As for detangling, I have found that Kinky Curly Knot Today to be the absolute best detangler I have found. It's a light leave in condish that she can use to help prevent tangles throughout the day and/or can use as a conditioner for detangling in the shower.