That's actually an exploit, you can over bid for your player. I'm not in position to judge and I don't think we should do it but as long he is your opponent you are also right and you have this right to know if he is clean or not.
You can visit the new help page, inside the game in the left down corner is a question mark, click or tap there and open a ticket.
Or easier use this link to fill a report: https://nordeus.helpshift.com/a/top-...=web&contact=1
From own experience I can say that once it happened to me and I asked them to check and I was right, it was a cheater.
Good luck!