Hello,
The Narita Express is wheelchair accessible (two spaces per train) and has a toilet large enough for a wheelchair.
You can buy tickets after landing and should have no problem getting on the next available train. If by chance the train is full, or the accessible seats are already booked, your friend would just need to wait for the next one. The train from Narita to Shibuya departs about every 25 minutes.
It is possible to book online, however(!) it may not be the best option as you likely cannot chose to reserve the accessible seating online (you cannot for the shinkansen). This is to ensure the seats aren’t taken by those who don’t need them. If your friend does want to try booking tickets in advance, he can try using the Contact Us page of the JR East reservation website. They will be able to give a definitive answer. 🙂
Hope this helps! And if your friend does reach out to JR East about booking the accessible seat in advance, please let us know what the reply is so the next person reading this thread will know. 🙂