Was Benedict Arnold friends with the British?

Not originally. Benedict Arnold was an an American hero and one of America's best generals. After the capture of Fort Ticonderoga he came within a hair of capturing Canada. At the Battle of Saratoga his horse was shot from under him and he was wounded in the leg by a British musket ball. After he was discredited (an event which remains controversial to this day), Arnold had little choice but to ally himself with the British, as a hangman's rope awaited him if he returned to the American side. The British made him a general in their army, but other British generals did not like him or trust him, as they regarded him a traitor to the American side.