Brittny Gastineau 'Attacker' -- It Was Mutual Combat

The man who allegedly viciously attacked Brittny Gastineau was himself badly injured during their altercation ... and his lawyer tells TMZ it was a case of mutual combat.

Marquis Lewis -- a famous street artist who goes by the name Retna -- was dating Brittny. He had checked into the London Hotel in West Hollywood a week ago Monday to recuperate from major knee surgery -- surgery that had just been performed.

Retna's lawyer, Michael Goldstein, tells TMZ ... Brittny came to visit a week ago Tuesday night and they got into a major argument.

Goldstein says his client was almost defenseless from the physical attack because he was unable to walk without crutches. We're told there was lots of swinging, and in the end Retna was also seriously injured -- his face and eye were swollen, his painting hand was injured and his ear was cut up.

Goldstein says, "Retna was not the aggressor." That said, Goldstein tells TMZ his client harbors no ill will, calling Brittny "sweet." He says, "They're cool with each other."

Goldstein says Retna and Brittny are in the process of settling their differences and he's surprised Brittny made a spectacle last night at paparazzi-riddled restaurants.