As someone who currently lives alone and has been known to get out the carving knife when she hears suspicious sounds (tongue in cheek), I'd be scouring the house for objects disturbed or hidden, and I would be doing it with a trusted family member/long time friend because it feels really uncomfortable putting youself in the shoes of someone who's trying to hide or find something in your home. Someone not so used to the scenery will be able to look at it with fresh eyes. There was a reason that they went in that house, and until she finds out what the reason is (innocuous or otherwise) it's not going to be a comfortable place to be.
There are all sorts of security options she could look into if she's planning on remaining on that property for the foreseable future, but whatever she does a great first step would be to immediately invest in key-locked deadbolts for all windows, change the door locks if she hasn't already (I thought people did this is a matter of course when getting a divorce

) and check the latches upstairs to make sure the unwelcome visitor didn't leave him or herself an easy entrance point for future visits.
Sounds like she's had a pretty bad time of it lately, good thing she has a great sis looking out for her