Can a woman follow the teaching in Matthew 18:15-17 and go directly to a brother if she has is bothered by a fault or trespass of his, or must she go to the men of the congregation to approach him? It seems to me the passage applies to all Christians not just men and that if would be better to try to deal with the problem privately if possible.
Nothing in the Scriptures indicates that a woman can't talk to a man or point out something that needs correcting or improving. The only thing you can't do is demand that a change be made because you said so. "A woman must quietly receive instruction with entire submissiveness. But I do not allow a woman to teach or exercise authority over a man, but to remain quiet" (I Timothy 2:11-12). Because of the "receive instruction" I take this to mean that Paul is talking about public instruction. However, the principle of not demanding authority over a man should be maintained.
If someone is in sin, it is the responsibility of the person who knows about it to go directly to that person. Who knows? The matter might get settled immediately or you might find out you were mistaken. It is only if the matter can't be settled that others are asked to look at the issue.