There are many people I retweet whom I have no connection to whatsoever. I like what they wrote and I think it's relevant to or interesting for my followers.
Generally, information that has absolutely no merit in a court case isn't allowed in the courtroom because (I presume) it can subconsciously alter a person's judgement of the proceedings.
Assuming a retweet is the full extent of their "friendship", this is the real-world equivalent of dismissing an expert witness because the defendant quoted them in a paper once.
Something like a tweet is context dependent. Some people tweet mostly to their friends and a lot of retweets are in fact indicative of a personal relationship. It will often be easy to estimate for a twitter user, but very hard to make objective enough to work as evidence in court.
Do they have just 12 or 12000 followers? Are they tweeting personal tidbits or politics & jokes? etc. These things can add up to an very informed guess.
In the U.S. The admissibility of evidence is based on relevance.[1] The modern formulation asks: does consideration of a piece of evidence increase or decrease the probability of some material fact being true?
A retweet is certainly relevant. The evidence makes it more likely that two people are friends than the baseline where there is no retweet.
