No he doesn't have to change all 4 tires. He can buy one tire to match the others. At most if his tires are worn he should change 2 and just make sure they are on the same axle (new tires on the back and old tires on the front or vise versa). The only time that the diameter of all 4 tires matters is when it is in 4 wheel drive and that is only for reduced traction situations where the difference in the tires is lost to the loose traction and not sent through the driveline causing trouble.
So tell him if his tires are good then just buy a new one to match the old in type and size. If they are worn then buy 2 and put them on the same axle keep the old one from that axle for a spare and go on with life. The only reason to buy 4 new ones is if he needs 4 new ones or is changing tire size like going from 32's to 33's.
The Final Salute
EVERYBODY is pro-gun, some just don't know it. When an anti-gun person is in trouble, the first thing they do is call 911 and demand that they send somebody with a gun!
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Trying to understand some people, is like trying to pick up a turd by the clean side.