Your friend isn't convinced her workout should include bone strengthening exercises and she question spending time and energy on them what should you tell her?

Bones and muscles together form the supporting structure of the human body. If bones get weak, muscle performance will be affected and vice versa. So I would tell her that bone-building exercises are as important as muscle-building exercises. The difference is that muscle exercise makes a noticeable aesthetic difference, so people prefer it, but this is only possible with a strong bone support.