Just use your head a little, and you should not get into a situation like this. And there´s simply no two words about it: the river was big enough, that he should have parked the bike, and walked first. Then I´m sure he would have looked far enough and seen, that it´s just not do-able at the moment. I hope other riders, who have seen this thread, will take notice. Swift water will always require you to be extremely careful.