What should you do when your supervisor is ill?
Since in a recent email your advisor has told you about the health issue, it is perfectly acceptable for you to sympathize/empathize with them and it would not be out of place to ask if you could do anything regarding your work to accommodate them. That said, if you feel uncomfortable raising the issue, you should be able to depend on your advisor being mature enough to tell you what he/she needs in terms of accommodations.

If your advisor had not mentioned anything to you and you wish to sympathize/empathize with them, then you need to be more delicate.