As a girl, I’ll say:
Do not expect her to change after a quick talking to. Let’s say a girl was dating a guy and she was annoyed/worried because he seemed to be slacking off in going to mincha. (The exact thing isn’t important, just that there’s something he does that she doesn’t like/believe is correct.) Should she assume that you will hear a nice shmuess from your rebbi and immediately change? Same in this case. No change should be made for anyone but oneself or it will never stick and will never give real satisfaction. Imagine (assuming, for the moment, that the problem is that her skirts are too short, just ledugma) that you somehow get across to her (already difficult to do without potentially being very offensive and judgmental but definitely possible) that the skirts are an issue that is holding you back in the relationship. She really wants this to happen, she thinks you’re a great guy, so she says yes, I’ll change. There’s a sale on tea-length dresses and calf-length skirts and she immediately buys out the store. If she is ONLY changing so that you’ll move from date four to date five, and then eventually to proposal and wedding, then what happens in a year (if that long) when the whole glow wears off, there are bumps in the road, and she thinks to herself, this is the guy I changed for? This change will be entirely tied up in externals and if the externals seem no longer quite as worth it, only resentment can come. Or, even if you are idyllically and peacefully married for many years, she will not feel that the new longer skirts are something that she wants to wear, something that she chose because it was intrinsically important to her- they will be something that was placed on her as a burden and from there, resentment can build up as well. Unless she can summon up real desire to change that has NO connection with the guy she’s dating, no change will last with good effects.
It is very possible (though only after you know each other quite well, as in mft23’s case) that you can broach this topic in a way that will be well-received. It will be difficult, though, and even afterward, you can’t rely on a bandaid solution to erase the issue from existence.
Good luck!