Youve got to be practical she probably knows this is not how a frum woman shiuld idealy be dressed( i dont want to go into halachic questions …) if shes been thru bes yakov the mussar is probably coming out her ears. And any mention about olam haba will go straight out the other ear. Smokers know they probably wil get cancer but it doesnt help y should olam haba? Keeping the tora need much more depth than that. We are not muslims who must sacrifice our life (or fun) purely for the sake of another world.
Informing someone of an issur is if he thinkz its muttar. Here is a totaly different issue. The are many possible factors influencing her actions the question is if you or anyone can change that. One major problem is realizing the importance and royalty of every jew. I heard that the british royal family are not permitted to be seen with sleeves above the elbow because it is not befitting a princess ( not because of halacha!) Giving criticism (i refuse to call that mussar) would certainly not help make her feel more royal or good about herself. However tbis high level of feeling like a princess is probably not the only problem. She possibly has a low opinion of herself and needs to fill that emptiness with fitting in building her self worth on what others think of her. You must also take into account the shalom bayis and how much love, respect and satisfaction she gets from that relationship that she may feel a need to compensate. Some of these issues u can help with perhaps with guidance from an experienced rav who unddrstands peopls issues even if only by giving a compliment and showing u rspct her for who she is ( witbout being patronizing) Maybe she is waiting for someone to notice her and show she cares. In summary as msilas yesharim makes clear in hakdama mussar requires extreme depth and chochma usualy much more than peopl realize.
Wishing u hatzlacha in reaching the heart of every jew.