I would say yes he can use it but probably should pull the prop or bag it (on the 25) showing that it couldn't be used.

If he's got the the trolling motor mounted on the cavitation plate he'll need to remove the prop. We've been telling people that if they have their big motor tipped up and a bag taped (preferably a red bag) around the prop, we won't have an issue. But, if he's got the large motor down and it's over the HP limit he can have a problem as the officer won't be able to tell which motor he is using.

The other option is if he has an old bow or stern trolling motor that he can mount on the boat temporarily, use that and keep the big motor tipped up and bagged.