Is it normal that my friend bothers me with her behavior?

Don't talk to the husband; talk to your classmate. Making the wrong move could ruin a marriage. She probably gets something from the stud, or her relationship with him, that is absent from her marriage (feeling attractive; witty flirtation). It does not mean she does not love her husband. Tell her you're worried about your friend (her husband) getting hurt. Alert her that her flirtations could have unintended consequences, like word getting back to her hubby, or her own reputation getting sullied.

I myself have been flirtatious with attached women, knowing full well that there was no chance of me getting between them and their partners. I also know that my attention made them feel good about themselves.

I felt the same way about a friend I shared a class with in college. It disgusted me. I knew she had a boyfriend who she had been seeing for a while, but I watched her everyday flirt with some 'pretty boy' throughout the whole 2 hour class. I felt really bad for her boyfriend. And I kinda felt crummy for not saying something, but a part of me felt like it was none of my business and not my place to say. But let me tell ya, everytime I saw her boyfriend come around all hugging and kissing on her and everything, I looked at how oblivious he was, feeling kinda bad for the guy. And I looked at her and thought, what a sneaky ass bitch! It made me think less of her as a friend.

You should probably talk to her about it when you guys are alone. Just you and her. But remember, it could be just harmless flirting. If she takes it any further than that, her husband has a huge problem.

It's quite disrespectful, but perhaps they have an open relationship.

I think you should confront her. Or tell her.
Probably both.

Tell him.

Yeah, it's pretty disrespectful that she flirts with another guy when she's married. It's normal to notice someone is attractive, even if you're committed, but acting on any urges is a no-no.

I don't think that you are wrong. It is frustrating to see someone blatantly break their commitments to others without a care for who sees. If their marriage is monogamous, then she is being extremely disrespectful by being so blatantly unfaithful and that is a very distasteful trait.

However, I wouldn't worry about it if I were you. If you know her husband and you two are friends, then I'd let him know about my observations, but if I don't know them and I don't know their situation (for all you know, they may not be monogamous, they may be swingers, he may derive pleasure from her unfaithfulness), then don't waste any energy being upset.