What should my friend do to expiate her sin, after she repented from committing fornication with men through anal intercourse?

What your friend was doing was forbidden with husbands, so how about with strangers? Indeed, it is among the greatest major sins and the most heinous of abominations. If she had died while insisting on practicing it, she would have died a disobedient, immoral, shameless woman. Now that Allah has bestowed upon her repentance before it is too late, she must be sincere to Allah Almighty in her repentance and remain steadfast upon it. And Allah Almighty, through His grace and generosity, accepts the repentance of those who repent, forgives their sins, and overlooks their misdeeds.