My friend takes biotin for hair and nail growth and she has seen results, but is it safe for her to take this supplement regularly?

Yes: Biotin is extremely safe. It is a natural vitamin found in a wide variety of foods such as chard, carrots, almonds, milk, eggs, berries, and onions. Quoting this fine article: http://umm.Edu/health/medical/altmed/supplement/vitamin-h-biotin "biotin has not been associated with side effects, even in high doses, and is considered to be non-toxic.".