It really depends on your hair and your climate. When I was younger I had very oily hair and lived in a very humid climate, so I really needed to wash my hair every day. I had friends and family that had drier hair and washing it that often would over dry it. I had one friend whose hair was so dry she conditioned it everyday but only shampooed it every third day. It all comes down to what your hair is like.
Washing your hair everyday is okay. However, you have to pay attention to the shampoo that you use. Go for one with a gentle formula and moisturizing ingredients. It is also good to use a moisturizing conditioner after using shampoo.
It depends on how dirty/oily your hair is. Many people can go without washing there hair everyday. If the person has dry, frizzy or brittle hair I would avoid washing everyday. Washing too much can dry out your hair and cause damage. If they have been doing it and they have healthy hair then it shouldn't be a problem.