It depends on the transformer. Usually scooter chargers, as laptop chargers, have a transformer that adapts the voltage to the charging needs of the device. For example, I’ve got a mobility device that accepts inputs between 84V and 230V. Japan is 100-110V, so I had no problem. This information is written on the transformer (the usually black rectangle box that’s part of the charger, as in laptops).
If the transformer only allows in the range of 220-230V it will charge extremely slow or it may not even charge.
Aside from that, there may be other issues with mobility scooters in Japan and using public transportation depending on the model. I cannot help you with this, but I’m sure that other members will.
I hope that your customer has a great trip to Japan!
I agree with Joan that the charger MUST say that it will work with 100 volts. If it is an UK only model , ie it is 230 ( or 240 volts) volts it will not charge your batteries in Japan. If it is an international model it will likely work.
See if you can post a picture of the charger information plate in the forum. We can tell for sure.
You will also need a mechanical converter for your plug or a different line cord, of course. Japan uses a small (compared to the UK!) two prong non polarized plug.
There is potentially a further problem. Many airlines will give you a problem with deep cycle batteries. They may require gell batteries. These are batteries where the liquid electrolyte is a gelled electrolyte that will not spill if the battery is broken open. Some airlines will accept liquid electrolyte but will remove the batteries and double bag them.