After six weeks of getting along, now they are fighting

Hi all, I described the situation in another thread here http://www.thecatsite.com/forums/sho...d.php?t=178822

Basically, I gave a female cat to a friend who already had a male cat. After a few days of hissing/spitting/swatting, they started to coexist peacefully. They even were sort of playing with other, and everything seemed fine.

Now, after six weeks, they are fighting with each other badly, and the female cat actually took a chunk of flesh (dime sized) out of the male's neck.

What could be the problem after six weeks of getting along? I don't understand what could have happened? Is there anything the new owner can do?

Thanks!
Journey

I take it that they are both neutered?

yes they are both neutered. Male is 4 years old, female is 9 years old.

Can the see other cats outdoors?