"To be on the bench" is a metaphor from sport's terminology. To be on the bench in, say, football means to sit on a bench and not play for a while, then get off the bench and return to the football field. He means it metaphorically, in a sense of "welcome back after a long absence".

"on the bench" and "off the bench" these are sports metaphors. For example, in football or hockey some of the team isn't playing in the game, they are "on the bench". When the coach replaces the players on the field some of them come "off the bench" and are back in the game. So the boss in your example considered the woman to be "on the bench" and not working with the team. When she came back to work he say "look who's off the bench", meaning "look who's back at work". P.S. - kind of a jerky thing to say to a woman who's just given birth.

Correct, « Off the bench » is a sports-related idiom. When a player is « benched » by his coach, he literally or metaphorically « sits on the bench » and does not play. In this context, the employee was « benched » or « on the bench » because she was not « in play » , which is to say, not working. When she returned to work she was « off the bench ».