The cassette is held on by a lockring, no need to press anything. Though you will need to get a special keyed socket to take it off.

How many miles on his rear cassette? If it's not nearly new, it's not even worth $2, let alone $20 - since you can't really be sure if it was maintained (cleaned and lubed, not run with worn chains). You'll wreck your chain in a few hundred miles or less on a worn cassette.