Does he have to have a face-to-face meeting anywhere during his application? I believe Seattle is five hours drive away.
Nope, no face-to-face meeting or anything - the application is processed entirely in Sheffield, so it's not possible for them interview him in person from 6,000 miles away
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The only thing he has to do in person is attend a biometrics appointment at a local USCIS Application Support Center, but that's just 5-10 minutes where they check his ID, stamp his biometrics confirmation sheet and take his fingerprints and photo. It's done by US immigration on behalf of UKVI, so they have nothing to do with his actual application.
Also, how do they take payment? Can he pay by card or from his account? I wanted to use my credit card from here.
The payment is taken online by debit/credit card at the time of submitting the online application. The fee will be quoted in US dollars (currently $1,513), but I think there may be a currency conversion fee depending on whether you pay with a US or UK card (someone mentioned a fee when paying with a US debit card, but I don't recall anyone else saying they had to pay a fee).