Answer this with a question - batter hits ball into gap, starts heading to second - realizes he's going to be put out, turns around starting back to first. Somehow his shoe comes off, throw comes to second baseman who tags the shoe before the runner gets back to first - is the runner out?
So what's the difference with the helmet? That isn't on his head...
Surely a had to be there type situation. I'm sure depending on which side you're rooting for the helmet either was or was not touching the runners back, but I don't believe you "should" take an out there... Beyond that - if the defense cannot make a simple throw, do they "deserve" the out?