The only exception is if he agreed to make two or more deals with multiple people at the same day or around the same time. Electrum could potentially connect his UTXOs since there are is no manual coin control option available.
If he doesn't care about privacy between trades, then just send all the coins he is planning to trade that day in a single UTXO to Electrum. The change from his first trade becomes his output for his second trade. If he does care about privacy then create entirely separate wallets, one for each trade, as then there would be no risk of outputs accidentally being used together and no risk of any change from the first trade accidentally being included in the second trade.
Still, this involves carrying more bitcoin than necessary to each trade, which brings additional risk.