Sorry but saving throws, steal checks and attack rolls all use a d20. The only time you'd use a d6 is if you were rolling stats at creation or for damage.
Look out Chaz, your Ti is showing.
And for the record, while it does rhyme with 'Like a G6' (do they have this song in England?) the statement is "Steal a Wallet from a guy. Roll a d6" which means she already made the Thievery check (4th ed... forgot what it was called in previous editions...) and is now rolling a d6 to see how much she got. Standard roll was something like d6x10 Silver Pieces if I remember correctly.
feux nerd-rage.