In PK movie, PK knows about Jaggu's boyfriend that he didn't cheat/left her in Belgium. But PK only read the mind of Jaggu. PK didn't know about Tapasvi's lies. How does he know that Sarfaraz still loves Jaggu in the ending?
PK did't know about Tapasvi's lies but he had power to know what actually happened by holding person's hand . So he came to know about the lies while holding jaggu's hand which Tapasvi told him on the video call .
Now coming to the Sarfaraz's point , He made that assumption which was made by considering these three situations or facts which actually happened at church on the wedding day .
- The boy who came to give the letter was unknown to Jaggu .
- There wasn't anybody available there other than a girl who was holding the basket with the cat .
- Nobody's name was written on the letter.
So PK came to conclusion by using this facts that letter might not from him it was a guess which could turned out to negative but considering Sarfaraz's image on Jaggu's mind, PK may have thought that this is possible which actually was .. :)
He shouldn't have known given the available data. He guessed and it turned out to be right. But he acted as if he knew the truth, which is a logical flaw. He should have acted like he is guessing. Given that he was guessing, it looked like it was a frivolent wager to prove that the Tapasvi is a shallow Sadhu. Had the guess gone wrong, both Tapasvi and his remote would have gone other way and the movie would have ended differently. Sadly, Indian movie goers dont worry about Logic & consistency