Is it unethical to take ideas from a pre-print to write your own research paper (though the author of the pre-print is acknowledged)? One of my friends is in a very curious position. He has been researching on a number theoretic problem for months but couldn’t solve it until recently after he has seen a pre-print in arXiv which attempts to prove the problem. But though he has taken ideas from the pre-print, his own article differs considerably from that of the pre-print. According to him, following are the chief points of difference,


My friend actually proved a stronger version of the problem.
He gave an outline to the proof of a conjecture that the author of the pre-print stated in the concluding section of his/her paper.


My friend has decided to mention precisely how much he had been benefited from the paper in the Acknowledgements section. Will there be any problem if he sends his article to a journal for publication?
He should just cite what he used or learned from the ArXiv paper and send it in. There's no real need to add an acknowledgement.