Are my friend's prayers invalid, and will he be punished for believing in the purity of pre-seminal fluid (madhy), and does the same apply to my other friend who follows the same approach, and are their prayers accepted or not accepted?

The opinion that madhy (pre-seminal fluid) is pure contradicts the relied-upon and preferred view in the Hanbali school of thought. Indeed, a consensus on its impurity has been reported, and its impurity is indicated by the hadith of Ali in the Sahihayn (Bukhari and Muslim) and other collections. As for your deceased friend, he was exercising ijtihad (independent reasoning), and it is hoped that he will be forgiven. As for the living, you must advise him and clarify the matter to him. His affair in the Afterlife is with Allah the Almighty.