While the crush itself is completely natural (source: having been a teenage girl who was always drawn to older men on TV, but knew better than to pursue any in "real life"; also simply overhearing and seeing young women/teen girls talk about the guys they are interested in) and not necessarily indicative of a problem, any inappropriate behavior is a huge one. No one, regardless of gender or age, should be made to feel uncomfortable by another person's romantic feelings.
Is she actually flirting with him and/or trying to get his attention sexually? Or does it remain innocent, simply wanting to spend time with him or impress him? How well does your friend know the girl and her parents? What is his relationship to the family? Does the girl seem relatively grounded in reality and normal in behavior otherwise? Does she ever use manipulation tactics or is her demeanor that of a typical naive teenager? Don't worry, I don't expect you to provide an answer to all these questions! ;-) But I think they are important for your friend to answer for himself.
If she is truly being inappropriate or your friend is very uncomfortable, my advice is don't hesitate to talk to the parents. And... while I truly hate to say this, he might want to make other people that are on "his side" aware of the problem, just in case the parents were to have a negative reaction that consisted of believing that their daughter is perfectly behaved/nothing wrong and your friend is the one being inappropriate. I know it sounds illogical, but it's one of those "it's the way the world works, unfortunately" situations that should be considered, for his own legal protection.
IF, however, it is simply a crush and the most she ever does is giggle around him or smile a lot, for example - as in truly "innocent" actions that aren't meant to be seen as pursuing him - I would let it go, perhaps distancing how much time he spends around her if possible. To bring it up will be a huge humiliation to the girl and may create an issue where there is none.
Long story short: your friend should go with his intuition in this situation. A crush on an older man isn't in itself wrong or even weird. It's her behavior and demeanor he needs to be aware of, and ultimately he needs to protect himself. And if she really is suffering some form of mental or emotional problem, taking this step to get her help could be life-changing or saving for her!
Good luck and, to your friend, my sympathies. It's a tough situation.