"How Hi's a Chinaman?" as a confused old workmate used to say. Maybe it's not very PC now, so I'll stick to " How long is a piece of string?".
My guess is they'll want at least 10% and then possibly some staged payments.
Every company will be different in costs (I hope he's taken 3 quotes from other companies for a job as big as this), and the contract he'll be asked to to sign will tell him exactly how much the whole job's going to cost, and how much they might want as a a deposit. Depends on the company - when I had my double glazing done, the company didn't want a deposit at all, the signed contract was enough of a legal document for them to start the work.