If you strongly believe they are cheating, you should probably ask a higher rated player what their opinion on their play is, after the game is over. Afterwards, you can also run it through an engine if you know how to analyse it and then, if you are stil convinced said person is cheating, report it to Chess.com. 
Cheating..
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I was just wondering what everyone's thoughts were on cheating. Obviously we all think it's terrible for both players, but what if you suspect an opponent in an internet game of cheating to save their game? What is appropriate action if any?
To be clear I don't suspect someone at the moment, yet. But the thought has entered my mind more than once since getting into chess over the internet..