Is this a red flag or truly something to be sympathetic of?

My girlfriend recently told me of a friend that has touched her (groped her inappropriate really from the sounds). Anyways, she was going to class with this guy and apparently he started to feel her boobs under her shirt she didn’t encourage it but didn’t tell him to stop only said she felt uncomfortable. Then she told her parents and everybody and nobody believed her about the story as she had stayed friends with the person. So is it concerning she stayed friends with someone who basically sexually harassed her but also felt strongly enough to tell someone they did that? Is this a self esteem issue or why would she stay friends with someone who did this? Any help? Opinions?
Is this a red flag or truly something to be sympathetic of?
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