Can I report on behalf of another?

Recently I saw someone posting a suspicious picture in a selfie thread. I suspected it to be a selfie from another user, so I screenshotted it and copied the link to that specific reply and asked the victim whether this was them using the screenshot. They confirmed. So I wanted to report it, but the entire thread was already gone. Also I cannot report it in https://similarworlds.com/support because there I cannot report on behalf of another.
Help me please.

Edit: [@admins] helpful people suggest me to tag you. Please help.

Edit2: Nuno will pick this up further. You all thank you for your help.