In reply to James Malloch:
Google like to be able to claim that they have good privacy options, so I'd wager there's a setting somewhere that disables search history, or sharing it between devices, either globally or on a particular device. But Google also don't want people to actually turn those things off, so it'll probably be a pain to find.
The easier option might be to remove his account from the Gmail app and check it using a third-party email app that doesn't integrate with other things like search history at all.
Edit: Got distracted before posting and beaten to it. But at least the advice lines up.
Post edited at 11:41