I’m just curious to know why you think your friend is “settling”?
Unless the future spouse is abusive, it really shouldn’t matter to you whether that person is good looking, makes a lot of money, speaks perfect English, wears the right clothes, knows the right people, combs their hair in a flattering matter, drives the right car, lives in the right neighborhood or anything else.
If the person is abusive, or an alcoholic or a druggee or a compulsive liar or cheater (and you can prove that) you should probably talk to your friend now and not wait until the last minute. If any of these things are the case, you should let your friend know that you are genuinely concerned for his/her welfare and then suggest that you wouldn’t feel comfortable being in the ceremony. But know that if you do that, you might lose your friend. But if your friend is reasonable, they might thank you for trying to help. It could go either way.