Allow an ally to reroll after seeing the result if you can reasonably explain how you could help them. The GM may require a skill check—or even disallow it—if the idea is a longshot (limit: 1 Help/roll).
Teamwork! Help as a Heroic Reaction is a fantastic way to bring teamwork, role-playing, and creativity into a combat encounter. Did your buddy fail? Maybe not! Think quickly and see if you can help them succeed.