Hrm. Despite my comment below, yes, I would agree that, if you're betareading for someone for the first time, it's best to point things out tactfully. Otherwise a beta can be accused of being a mean bitch and killing inspiration, and the drama from that (and the associated hordes of e-mails where your writer is questioning his or her writing ability) is TIRING.

I did a verbal beta for my BFF, whom I've known for 12 years now, and I still had to tell her multiple times that I. Don't. Hate. Her. Fic. (She has issues, unrelated to me, which I inadvertently triggered. OMG.)