what happens to a toyota 4 runner when the muffler is removed and replaced with a straight pipe

It probably won't do any damage. The muffler might provide a little back pressure and the engine might use a little more gas but no real damage. Traffic tickets will damage his wallet.

The removal of the muffler should not do any damage. The removal of the cat converter would be another story. He should have it replaced with another more "straight through" muffler if he wishes to have louder sound.

I wont do any damage.

If your in an area where emission standards are in effect you need to check if this violates emission testing! Back pressure is reduced so the engine breathes better but puts more contaminants and sound into the area!

Depends where he took it off from. If there starts to be a issue , its probably the o2 sensor