If one of your co-founders leaves, it can be an incredibly rough time for you and the company. So here's the core things you need to do, to ensure this isn't a fatal issue:

1. Handover Docs

I know it's boring, but you need to make a list of the co-founders responsibilities, and how they're completed. Try to make this as authoritative as possible, you don't want to have to call them at 2am with some urgent problem.

2. Compensation

You'll be handing over these responsibilities to other folks, so those people should get more equity and more cash if possible. You need them retained and engaged.

3. Job Posting

It's easy to dwell on the loss, but instead get started on hiring right away; put the job posting up everywhere you can and start meeting candidates. Momentum helps.

Best of luck out there.