First decide if this is something you want to do. If not, say 'no.'
Do you have any idea what this person's expectations are, as far as cost and time frame? Be sure and sit down and discuss cost and expectations before you agree to do the work. Then put what you've agreed on in writing , both of you sign it, and ask for half the cost up front.
As to what to charge, what do you consider fair compensation for your time and skill? You could base it on time, estimate how long it will take you, and calculate the total based on an acceptable hourly wage, or you could settle on a sum based on amount charged per square inch and pattern difficulty. I don't know would be typical for that, but I'm sure others will be able to make suggestions.