Asking for a friend, I will try my hardest so it doesn't appear as a "what choise to make" question, sorry about the long post but it is an interesting scenario:
The setting is a highly political government agency in the public eye. They do a great job but the office politics makes it a snake pit to work in.
Two ladies, lets call them Anna and Bekky, they also have a boss lets call her Charlie.
Bekky is newer in the job than Anna but has shown great talent at it and works very hard.
Bekky and Anna used to work together at the same location. After a while Anna got a promotion as an "acting manager" and went to a different location, less desirable area and the commute is longer. Due to the nature of the job the work is also much more difficult since the public they have to deal with in this area are different. In short no one likes it there.
Bekky meanwhile also got a promotion and became acting manager in the first location where she used to work together with Anna.
Anna then (since she was on the waiting list for promotions longer) got a promotion and will no longer be "acting manager" but will be "manager" and she has been offered the role Bekky was filling in for as acting. Anna is thrilled to be coming back to her area and to work with her friends again.
Here's the twist: Anna is pregnant and hasn't told anyone yet. She will be coming in literally for 3 or 4 months and Bekky will be bumped right back down because of that. Charlie, the boss, doesn't know Anna is pregnant and is hoping Anna can bring some stability to this team that has had 4 "acting managers" in the last 2 years.
Bekkie knows Anna is pregnant. Bekkie is also very resentful that she will lose her spot as acting manager and maybe even get pushed back down or possibly even get sent to the undesirable location Anna is coming from. Bekkie is especially resentful that Anna will be going on maternity leave for at least 6 months for which she could have stayed in this spot.
Bekkie's friends are telling her to tell the boss Charlie that Anna is pregnant and only coming back for a couple of months. She is considering it at this stage but isn't sure what repercussions might arise from this.
Would Bekkie be better off letting Anna just come back for 2-3 months and accept her fate or say something? What would be the potential harm?
tl;dr manager pushing acting manager aside when she is about to go on maternity leave and she is hiding she is pregnant, what could be the damage if acting manager tells the boss what is happening?
Edit: What Bekky wants: Anna already has the position, Bekky just wants Anna to stay put for 2-3 months so she can cover Anna's absence as well as "acting".