Most good small business sell at 3-4 times net earnings. At 0.42 times net earnings... then go for it before they change their mind. You'll make your money back in 5 months.
However.... if it's too good to be true... it usually is.
300k for 1/3 share of 180k net per month... works out to be about 0.42 times ebit. The two months it'll take you to organise all the legals and financials, they would have made the 300k.
Is your mate a Nigerian prince with access to some gold?