Just like your friend said - let them go at it. I have two rotweilers. The male is pretty nice and the female is good with most males but not females. It depends on which dog is feeling 'alpha' at the time. I met my friend who lives across the street through our dogs. They always barked at each other so one day we took em to the park to play. We both knew they would have to have it out first because they both thought they were better - and so we let them fight (but were ready to kick em apart if it got too crazy) it lasted a few minutes and then they were playing fetch together and are best friends now.
Introductions like that suck but it has to be done if you want to keep both.