What voltage does it use?

No

Rewiring a toaster is an invitation to having a fire or electrocuting someone.

A British mains toaster is designed to run on 230V. Unlike something that would have a motor in it, it's pretty straight resistance. To run in the US, you would just need to use a step up transformer (such are sold specifically for this purpose). If there are three pins on the British plug, another option would be to connect it to a US 240V circuit directly (my neighbor has such in his kitchen as he is Swiss and has some appliances he brought over).

However, by the time you do any of that, you would probably be better to buy the appliance already set up for US mains. Smeg toasters are available that way.

There's a hack that can do this. You'd tap the elements at their midpoints and connect the halves in parallel. In practice, doing this safely is a challenge for many reasons, so this is mostly an academic exercise.

Installing a 240v receptacle in the kitchen might work. A warning though, US 240v power is not the same as EU 220/230/240v. EU power has one side grounded. US 240v doesn't. Depending on how the toaster is switched, you could end up with elements in the toaster that were hot to ground, even with the toaster off.

This is why I suggested he not connect a non-grounded appliance to a 240 US circuit.

You also have the thermostat or whatever controls the thing to worry about.