Should I tell friend about her child's behavior?

Don't know for sure, don't say anything.

A child’s thinking behind this type of behavior is that “No one will get hurt and the store has a lot of money.” They rationalize that they need to have this stuff in order to be accepted. They might say, “My parents won’t allow me to buy clothing or makeup like this, so I have to steal it.” But remember this: It’s our job as parents, teachers and therapists to strongly defend the concept that stealing is wrong. Tell your children this: “Stealing is wrong for two reasons: It’s illegal and puts you at risk of being arrested and prosecuted. It’s also hurtful because when you take something that doesn’t belong to you, somewhere, someone down the line is being hurt.” Make it real to your child by explaining that if they shoplift cosmetics or video games, the company adjusts its price upwards to insulate itself, and all the rest of us pay a little more for it because of it.

If your child is caught stealing, in all cases, there needs to be meaningful consequences for the behavior. To you as a parent, the most important aspect of your child's decision to steal is the way of thinking that preceded the stealing. She should pay whatever the consequences are for stealing, and also write an essay on how she justified it. Ask her, “What were you thinking before you stole this?” Remember this: It is in the examination of the justifications and excuses where the true learning will take place.

Certainly consequences like making her take the stolen item back to the store, apologizing and making financial amends are all very good parts of the equation. That kind of accountability can be very productive in deterring future stealing, if accompanied by an examination of the faulty thinking which drove them to do it. You also might give them the consequence of, “You can’t go to the mall for two weeks. Two weeks of no stealing.” If parents ask me, “How do I know?” I say “Don’t worry about it. They need to get another chance. You’re not there to be a cop.” Always give them the chance to earn your trust back.

Read more below:

Do whatever you think is the right thing. If you feel that your friend's child is behaving really unlike his/her self, you should say something in case something is really wrong. Otherwise, you might not want to get involved. :) good luck!