For a hunting type shooting situation closing the dominant eye would be a good choice if shooting from the 'other side' is not an option. Since only one shot or two will be fired. But if shots will be taken on a regular basis,such as sighting in, it is better to keep both eyes open and block the vision of the dominant eye. A card taped to the scope or flip down blinder on glasses.
Holding the eye closed causes facial muscles to fatigue and done for an extended period of time can effect the aiming eye.
Experienced competitive shooters routinely some sort of blinder but as said earlier they are firing multiple shots and over an extended time frame. For the OP have your buddy try closing the dominant eye.
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