I suppose it depends on the level of racism.
Apart from about three or four friends, I suspect that many of the people I know have some views in relation to race which I would disagree with. One of my good friends has very different political affiliations from me - a committed Conservative with views often quite opposite to my own with regard to immigration, integration, etc, etc. If she expresses a view I don't agree with, I just calmly tell her I don't agree, and why. Then we usually drop the subject. There are things she says that I don't like but she is not a "bad" person and has a social conscience and does a lot for other people - visiting elderly, housebound people, helping in a club for older people, working in a hospice shop.
If people say something which I disagree with, I generally give my own point of view and leave it at that. If something which I consider to have undertones of racism is said, I challenge it. It would only be if I started to hear hate-filled opinions and outrageously offensive stereotyping that I would discontinue the friendship.
Having said that, if I met someone for the first time and got on with them well but, in the course of the conversation also discovered they had racist views, I would not particularly want to meet that person again.
I'm wondering if people are being deliberately obtuse in objecting to the Black Lives Matter slogan. It really is quite simple. Of course all lives matter but there isn't a great deal of evidence to demonstrate that black lives matter as much.