assalamu alikkum
first of all i am sory to ask a quetion like this from you on behalf my friend.
he likes to wear ladys panties. he like it very much.before merriage he used to wear his sisters panty. now he is wearing his wifes panty. no body knows about this but he is used to tell me all the time. he used to tell that he feel same on him self, also he tries to give up but still he coud'nt. is it a big sin that he is doing please answer.
jazakallah

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

It is forbidden for a man to wear the clothes that are peculiar to women, whether this is regarding underwear or outer clothes. Abu Hurayrah said: “The Prophet cursed a man who wears the clothes of a woman and cursed a woman who wears the clothes of a man.” [Abu Daawood] This Hadeeth is general and it applies to both the underwear and outer clothes.

It is not a condition for a person who seeks to imitate the opposite sex to have the intention to do so; whoever imitates the opposite sex is dispraised whether or not he intends to do so.

Since the one who does so is cursed, then this is evidence that this act is one of the grave major sins. Therefore, your friend is obliged to take the initiative to repent and stop doing this act. For more benefit, please refer to Fataawa 86527 and 87903.