the 139 is capable of being OEI at less than 50kts or in the hover, but you can only assess that by reference to the PI and your drop down height
Would not pre flight planning also indicate at what Weight, Altitude and Temperature they might be OEI capable?
One would contend that ye olde Rotorcraft Flight Manual would be able to inform them that, with 2 POB and at close to minimum fuel, they would expect to have OEI HOGE performance