When you don't feel comfortable doing something, it usually is a good time to hit the brakes.  You said you did "there" taxes as in a joint return last year?  If that is the case he can always ask you for a copy of last year's return so then he could provide the number to you.  If you prepared their returns individually, I think you are going to want to have him get that number rather than you grabbing it off of her return.