Let's face it, no wants to be the bearer of bad news and have to tell a friend that their fashion sense is not working. On the other hand, in order to be a really good friend, you have to tell your friend when they are leaving the house not looking their best. After all, if you let you friend go out in public looking like a slob and a stranger tells them, they will end up being upset with you for not informing them of their fashion disaster. There are ways to tell a friend when something is working without being critical, you do not have to say, "Well, that outfit shows how fat you are!" You need to use tact when it comes to telling your friend when their fashion sense is off.
The best way to tell your friend that their outfit is not working is make sure you do so with positive remarks as well. For instance, your girlfriend is going on a first date, she gets dressed and is wearing an outfit that looks like something your grandmother would wear. What do you say? Obviously, you can't say, "You look like a granny in that get up!" because this will hurt her feelings. Therefore, in order to be tactful, begin with the positive aspects, such as "You have the most beautiful hair and your makeup looks flawless!"
Next, once you have made a positive comment on something, say to your friend, "Do you remember that dress you wore a few months ago? That was smashing and I bet your date would go wild over that!" Of course, her first reaction will be, "does this outfit suck?" You then can say, "No, not at all, but remember this is your first date, so how about trying on some different outfits and I can help you so that you look drop-dead gorgeous!"
If your friend has no fashion sense at all, suggest you go shopping with her and help her pick out something for the big first date. Bring her to sections of the store that you know would work for her and allow her to try them on. Complement those outfits that bring out her eyes, accentuate her curves, and just look very classy. Because you are throwing so many positives at her, she will not have been offended by your comment on what she had planned to wear.
Women tend to be especially sensitive when it comes to their fashion sense. They often will ask a friend, "how does this outfit look?" but what they are really asking is, "do you think I did a good job putting this together?" Always start with a positive, drop the comment that it is not working (nicely!), and then end it with several compliments, that way what your friend "remembers" is the positive comments. If you just put out a negative comment ("you look fat in that"), you may not be friends with the person for long because she will be offended.