Haven't read other replies . I'd absolutely instinctively challenge them and have. I'm a female but that makes no odds if you're a human you must have that instinct to protect another person , male or female , even causing a fuss would be enough .As it happens I've been in the situation and I didn't hesitate ... it resulted in something or nothing , who knows what it may have led to .
Once when I was walking my elderly dog, a lose staffie appeared from nowhere and attacked him. I screamed and yelled for help at the top of my lungs and this wonderful bloke came running and got the staffie off my dog. Luckily my dog wasn't badly hurt, only a cut under his eye. That man was my hero !