If people are asking to be beta'd, they *should* be ready to receive criticism. If they have multiple betas, they'll see whether your crits resonate with the others. If they don't think it makes sense or it doesn't match the rest, they might not use it.
I think the trick with betaing is knowing the difference between saying, "this is what I see is wrong," and "this is how to fix it." I do sometimes give examples of how things could be fixed (how I would fix it, but with major disclaimers), but in the end, it's the author's place to fix it. Sometimes, "OMG, I hate your love interest" comes down to one piece of dialogue or a character quirk that can be easily changed or cut, for example (this has happened to me). "You need more conflict" can be more helpful sometimes than, "add this and you'll have more conflict" because if the writer doesn't agree with the "this," they'll reject the whole critique instead of focusing on the important info that they need more conflict. As examples.
When I get critiques I don't like, I kick and scream and cry in private for a few days, then eventually go, "Oh, no, I know what I can do about that" and revise. "Eventually" used to take longer, but it's much shorter now.