How long would you wait to break up with someone you lived and worked with?

My boyfriends sister has asked if she can stay with us as she is planning on breaking up with her boyfriend who she lives with and works with. This was a few weeks ago now... I'm happy to provide somewhere for her to stay while she sorts her situation out but I can't help but feel bad for the guy who I assume is none the wiser that she is planning the break up. I also find it odd that she feels the need to plan everything out before even having the conversation with him but they're still going to have to see each other at work...

Part of me is waiting for the "nevermind I changed my mind"

How long would you wait to break up with someone you lived and worked with?
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