Cooper had known about the leg beforehand, but didn't know much about how it happened. She wasn't there at the time, and she didn't want to make him uncomfortable by asking a bunch of questions about it. As far as she knew, it was the result of a machine going haywire; part of that was assumption and part of that was infered from what she'd heard on the grapevine.
She didn't know how to react when Marcella made such a big deal over Max's leg. In a weird way she was proud of her friend, even if the reaction was mildly inappropriate; Marcella wouldn't have done that if she wasn't a devoted doctor and humanitarian. She wasn't sure if it was her place to step in and tell her to ease up, but she doubted so. In the end she stood there awkwardly frozen next to Lila, watching the scene unfold.