Which petition? I-129F for K1? Or is he going to get married on this visit, then file the I-130 for his wife-to-be's CR1/IR1?
If I-129F, believing that it will be approved in around 6 months is delusional. Current estimate is around 15 months, just for the petition stage, excluding wait time for consulate interview.
If I-130, 6 months for petition approval is possible, though very optimistic. 12 months would be more realistic. But who knows, your friend could get lucky.