I think Jassi needs to tell about Kohinoor to Purab. I don't think purab needs to say anything about the diary to Jassi. Don't they usually say that if there are somethings kept quiet and does not hurt anyone then don't say a word about it. So even if Purab read the diary it didn't hurt anyone but if Jassi doesn't tell him about kohinoor and she gets in trouble with the law then she will hurt a lot of people. So she should tell that to Purab.
I have no problems in saying that you have made a very good point.