Sanhedrin chapter 3 talks about categories of people who are disqualified from being judges or witnesses in a case. They include some people who are never trustworthy, various relatives of the litigants, and friends or enemies. About this last category, the mishna says:
וְאֵלּוּ הֵן הַפְּסוּלִין: ... הָאוֹהֵב וְהַשּׂוֹנֵא. אוֹהֵב, זֶה שׁוּשְׁבִינוֹ.
A friend or an enemy [is disqualified]. “A friend”: this is one’s groomsman. “An enemy”: anyone whom he has not spoken to in three days because of anger. They replied: “Israelites are not suspected of such.” (3:5)
I looked in the g'mara to see exactly what is meant by "one's groomsman", thinking it meant any time after the designation and wedding, but instead found a discussion about whether the person is disqualified for the seven days of sheva b'rachot or just the first day.
That's rather more narrow than I expected!
Legally speaking, do we really have so few friends? In my own life I can think of people who I'm much closer to than my father's sister's husband, yet the relative I might have no contact with is disqualified while a friend I spend lots of time with, or one who is a business partner, is not assumed to be biased in my favor.
Did this category get broadened in later halachic literature? Who, besides one's groomsman in the week of the wedding, is considered too close a friend to be a judge or witness per halacha?