Do you remember that scene from The Notebook, when Allie's mom is trying to tell her that Noah isn't right for her? Allie's mom assumed she was doing the right thing because she thought she had Allie's best interest in mind. Turns out, she was wrong. Unfortunately, real life isn't exactly like the average rom-com. Usually, when your mom or friends are telling you that a guy isn't good for you, they're typically not going to be good for you. It's hard for you to see when a guy isn't treating you the way you deserve to be treated, but it's even harder to listen to your closest friends when they're trying to break the news.
There comes a time when you'll have to be that friend. That time is when your friend starts telling you how the person they like doesn't always acknowledge them, starts making their self worth dependent upon someone else's opinion or just doesn't seem that they're able to be themselves anymore. Even though all of these things will be happening to them, they'll always follow up complaint with some type of compliment, thus reassuring themselves that their "significant other" is worth the trouble.
Being their friend, you know that it's not in their best interest to keep seeing this person. In fact, if you don't intervene soon enough, it might get to the point of no return and your best friend will start drifting away until they're not even your friend anymore.
No one else is going to take on the responsibility of telling your friend that she's settling. If you think that this is an awkward conversation for you to have with her, it's going to be extremely uncomfortable for someone who is not as close to try to approach the subject. Friends have each other's backs; they break the hard news to each other because no one else will.
Don't let your friends settle because you know they're worth so much more than that. You, more than anyone, see how much of an amazing person they are. By settling, your amazing person is losing a little bit of their luster. They're not with someone who is going to bring out their amazing qualities or potential, which isn't fair to anyone in that situation.
Having this talk with anyone is nothing less than incredibly difficult and unpleasant, but it's necessary. There is a chance that your friend doesn't listen, thinks you're being absurd and ignores you for a few days. There's also a chance that you help them come to their senses and find someone who is better for them. This is worth the risk. Warn them before they start a potentially disastrous relationship; they'll thank you later.