I realize that honest mistakes happen but this dude, friend or not, deserves what he gets. If he's trying to be a pro, he should act like one, multiple cameras, batteries, memory cards.

If he was volunteering or doing this for free, then it's the couple's stupidity in not having anyone else doing it. But if he was contracted to this and let something as careless as losing the only memory card with any photos on it, then he should not be a photographer and should be resigned to having some difficulties ahead.
Sorry to sound harsh. Hopefully, he'll find it and learn from his mistake. But he should have been prepared and never let himself end up in such a position to begin with.