Vibrato is something that can come very naturally to some, others mistakenly push too hard for it. If your friend is looking to do musical theatre, then vibrato is the preferred "legit" sound.
There can be all kinds of vibrato problems- shaky, scratchy, etc. With proper training and diligence, it can usually be repaired. Most likely, her issues come from weakened muscles, or the way air passes through her vocal folds. Doesn't sound like she has anything to worry about.
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