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I think there's a chain of events that begin with talking to the dude at the lake at certain time, and then gaining access to the safe or a file on the computer that has incriminating images of her you can blackmail her with or give them to her to get on her good side. Hopefully someone else can give a better answer

As Ainz advised there's a series of events you may complete. You first need to unlock the lake, you should have got the interaction with Sally about her lost keys in the locker rooms, then, speak with the dude in the lake, you can find him in the morning. Ask him if he has found the keys. Once the dude find the keys he will later speak to you about the old director and he will take you to the point where he found the body, then you will find his wallet with the code for the safe. This may take way too long depending on your interactions. 

To get the computer password, speak with Chang to get her favor, she will ask you to get her a assistant, then speak with Samantha about to hire a new lab assistant, you will get 2 applications and 2 interviews, the choice is yours. Once you hire one, speak with Chang and she will give you the computer password, to the last diary entry. 

Good luck!