What is the ruling regarding someone whose hand, wet with sweat, was touched by the impurity of madhy (pre-ejaculatory fluid), and then he touched another person's phone with it? And is it obligatory to inform him of that so he may purify his phone?

If a person confirms the presence of madhy (pre-seminal fluid) on his hand, and it then mixes with sweat, the impurity of the madhy spreads, contaminating what his hand touches, such as another person's phone. The one whose hand became impure with madhy must inform the phone's owner about the impurity so that he may purify it, as this falls under offering sincere advice to a Muslim. And whoever prays with the impurity of madhy, either forgetfully or out of ignorance, his prayer is valid according to the soundest of scholarly opinions.