I've had experiences where a friend of mine has a partner that I disliked. So what I normally do is mention the behaviour that they did to me that made me feel a kind of way like "when I met your boyfriend, he spent the entire lunch on his phone, didn't ask me a single question about myself, and barely looked up to respond to anything I said, that made me feel awkward and like he didn't care about meeting me" - and then I add a charitable interpretation as well "maybe he is socially anxious" - but at least now my friend knows exactly WHY I feel a certain way, and yet I've left the door open to being proved wrong.

In this example I turned out to be right, but I also didn't alienate my friend by just saying "I hate your bf", and I opened up the possibility for change on the bf's part as well in case his behaviour was an accident.