A skeptic has claimed to me that the Christian saying "Do unto others" (what we call our "Golden Rule") was actually plagiarized by the Bible writers from something originally taught by Confucius. Are you familiar with this? I doubt this is true, since no source could be provided, but I have to ask.
It is fairly likely that Jesus was not the first person ever to say something similar to "do unto others as you would have them do unto you." I do not think that the veracity, reliability and authority of the Bible is in any way reduced if this was not first said by Jesus. The important thing is that he said it. Jesus did not claim at the time to have been the first person on earth to conceive of this principle. He may have been the first, and, unless your friend can produce evidence to the contrary, it is fair to say that Jesus is the first one we know of to say this.
Having said that, your friend clearly does not know what he/she is talking about because there is absolutely zero chance that Jesus plagiarized this saying from Confucius for two reasons:
1. There is no record of Confucius saying this.
And even more convincingly,
2. There is no evidence at all of any direct interaction between Chinese culture and Palestine in the first century AD. There may have been some very tenuous interaction between Israel and India, but surely very little if any at all, but the idea that Jesus would have been reading the writings of Confucius (which do not even include the saying) is way out there. I have heard of no evidence whatsoever of Chinese influence in Palestine in the first century.
The person who made this claim is simply throwing mud for its own sake and has no legitimate information whatsoever to back his/her ridiculous claim. If one claims plagiarism, one must present evidence for such plagiarism or ethics demands the claim not be made. To speculate plagiarism apart from data is to behave unethically.