I can give you my thoughts on your second question: You can start a relaxed conversation with your wife about this, say your daughter said something that made you wonder and you'd like to learn more. I suggest that you not use words like "obsession" and "infatuation" in that first conversation. don't make prejudgements, just ask.

If her focus on a celebrity could endanger herself or others, you have to get professional help.

If this is a matter of a crush, an enjoyment of entertainment media about this person, doesn't sound like that would be a problem. Nor necessarily would be a fantasy about the person, unless it interferes in her relationship with you and others.