check your trading history is better suggestion, you can see if all your open trade is done, it could be there are seperate amount that still in pending/not fully trade.
If all your trade history say different amount with bought and the sell then you need to contact binance support, before that make screenshot with all your history will better for strong evidence.
I believe, this is his case here. Check all his trading history or even the My Trades tab.
Also, as what others are also suggesting, check via desktop/laptop as you can see it better.
And if he thinks that the problem is not from his side, he can directly contact their support.
They have the support button that he can always use. Don't think that he will be screwed by binance on this.