Depends on if she's senior to you or not. If not a senior, I'd ask if she was feeling stiff today because something hurts, or the ukemi looks unfamiliar. Hopefully a positive answer in there somewhere, then reassure that we'll go slow and don't worry, I won't throw you hard/fast/etc ... if she's quite junior, perhaps a hint or two on how to accomplish the fall safely.
If she's senior, it is a lot more tricky. Still the same opening line, with more emphasis on the stiffness rather than the ukemi (yeah, it's the ukemi, but a senior is definately not going to like admitting it)... trot out that old standby "I'm working on XYZ and it would help if you could kind of flow with me here, I'll be going fairly slowly and won't really be throwing you hard, just want to get the rhythm down, etc"
I've had the best success with scared stiff ukes by going painfully slowly, being really soft and gentle, and making sure I keep good, smooth constant contact (no sudden moves or jerks).
What I do not do is let uke skip the ukemi. I tell beginners how they can let go, get low, take a roll on their own, but that they will fall... otherwise you end up with more advanced students who can't take ukemi, which to me is a real contradiction in terms.