Does the reader need to know the info presented in the flashback? Does the reader need it at that moment to understand the action? If you say yes to both, than use the flashback. However, keep it tight. (If you say no to the second, figure out where you really need it.)

Corollary questions: Does the character who didn't experience the events in the flashback need to know? Does that character need to know at that moment of the story? This will help you decide how to include the flashback. Maybe the character can think about it without explaining it or only explain pieces.