But it is not perceived by the partakers as harmful at the time.
Anyone may in retrospect, come to regret any act of previously consensually mutual sex. Anyone. You don't need to have slept with 101 men to have regrets regarding sexual encounters, that goes as much for sex within the bounds of traditional matrimony/monogamy as it does sex for money, or sex within a group. But it doesn't make, within a society that is functional benign regarding porn and is increasingly morally relativist, make the sex itself more or lesser 'wrong'
It's possible she may later in time choose to define herself according to a different, less sexually relativist world view. For e.g. she may become a mother and then choose to avail herself of a world view in which conservative sexual values are now useful and advantageous for safety and continuity and then reimagine her previous choices to fit her newly acquired values. And THAT spiritual(?) incongruence may then cause mental distress. Yes.
But as it stands, she has merely had more sex to more men in a single session than the norm. It is neither worse nor better than sex between merely two people.