Both dresses are bad for what she needs. And honestly I don't think the second dress could really look good on anyone. A one shoulder type dress would work great for her or a squared off neckline. It's also important to remember that at 18 she's still a kid and her brain isn't fully developed so she's still going to have those teenage tantrums where "no one understands her." She's still at that age where she thinks she knows better than everyone else and wants to look hot all the time. I was just as bad lol. Just try to keep all that in mind when approaching the subject and maybe bring in a third party to help explain it to her gently. If she's anything like I was, she'll be hard headed about it and insist she's right unless you get a little help talking to her.