A friend's child was just diagnosed with broken ankle, is that a serious condition?

Varies a lot. Kids with growing bones can have lots of variation in the significance of the "fracture lable. The ankle ligaments are a bit stronger than some of the growing bone, so a kid may pull off a chip where a mature adult would get a sprain. They can get a fx thru a growth plate that could create problems, but as a whole, they tend to heal quickly under proper supervision with good outcome.
Potentially. Depends n the specific nature, location and degree of displacement of the fracture. Some require surgical fixation while others need time and a splnt. Get an expert opinion quickly so all your options are available.
It could be. It depend on the kind of the fracture, how involve the bone and the growth plate. Most of them healed with no problems and rarely could have growth bone issues. Thank you.