Some Jain friends say that Thirumalai is a Jain temple; is there any source from Purana-s to prove that it is a Divya Desam of Emperuman?

Not just one or two but 13 Puranas have stated about the glories of Thiruvenkata malai. There are various proofs such as the hymns of Azhwars, Acharyas, shlokas, and keerthanams of many saints on this Divya-Desam. There is no proof that Jainism existed at Thirumalai, it is just a baseless argument.