
The victim, a 40-year-old Italian man, was taken to the Chon Buri Hospital after the assault in front of the Lotus shopping mall in Tambon Samed at 10pm.
His Thai girlfriend, 25, who was a pillion passenger, told police that he had suffered from a broken leg earlier and could not ride fast.
She said they noticed that a Honda car was trailing behind their motorcycle, so they tried to avoid it, but the car stopped in front of them.
She said an angry Thai man, who appeared intoxicated, got out of his car carrying a hammer. He accused her boyfriend of driving drunk and hit his right arm with the hammer.
She said she fought back with her boyfriend's leg crutch, and the Thai man fled back to his car and left the scene.
Police plan to check footage of security cameras in the area.