If he has a 2WD Explorer they should work fine.
If he has 4WD it gets a little more complicated. Generally with 4WD you want the same brand and size on all 4 wheels to ensure the total circumference of the tires are the same.
That's especially true if his Explorer has Automatic 4WD.
The Automatic 4WD system has speed sensors on the front and rear output shafts of the transfer case and automatically engages 4WD if it senses a speed difference between them, usually caused by rear wheels slipping and spinning faster than the front (snow, ice, rain, etc.).
If the measured circumference of the tires you're selling aren't the same as the ones already on his truck there will be a constant speed difference causing the truck to continuously engage 4WD. Not a good thing, especially on dry roads.
