If remember correctly, Guinan warns Picard to leave almost as soon as they get there. We then go to a commercial and when we get back Picard's decided to go exploring. Who knows what was said by who in the meantime? But I'll give Guinan the benefit of the doubt as Picard has proven to be quite stubborn when he wants to be. And once he's made up his mind, what can she do? (I mean, he's just ignored a warning from a person who is apparently beyond friendship and family - this was a man whose mind was not going to be changed.)
I also don't necessarily think Guinan knew the Borg were in the area - she just knew that
that part of space was not a particularly friendly place for lost Federation starships.
BTW, I don't think Guinan was ever used as effectively again as she was in this episode.