My friend needs some help?

My friend was called a slut. She is totally NOT in any way, she's never even kissed a guy. She's always nice to everyone and has no reason to be called names she's not like literally she's the sweetest person EVER. Now, she was called a slut and later found out. I tried to defend her (she wasn't there at the time) and the accuser rolled her eyes at me. My friend later found out about these rumors going around. She doesn't know that I was there let alone know that I tried to defend her, and she doesn't know that I know about these rumors. I feel really bad for her and want to show her my support what should I do for her?