How old are the kids? A teenager I would assume to be autonomous and confident enough to make the decision on his own. A 7-year-old, not so much.
And how close to the house was Friend standing? That is, could he have reliably run back inside to ask his mom real quick, then jumped into your car without delaying anyone too much? Or could you have pulled up next to him and quickly called his mom yourself? I hate the thought of a kid standing out in the weather we've been having lately. ::shudder::
I think, after the fact, you could definitely call his mom and ask for a standing agreement. Though, even with the agreement in place I would still call or text his mom real quick as you're getting ready to leave the house each time you intend to pick him up, just so there's no chance of the kid falling through the cracks because his mom thought the weather was "bad" and you didn't.