Background
I've been increasingly unhappy with my workplace (you can look at my question's history for context if you wish) so I've been looking for jobs for the past 2 months with great success. I'm essentially already one foot out the door (still working hard obviously). For the entirety of my tenure at the firm, my co-worker (who has been there for twice as long as me) has also been unhappy; but unlike me he was not initially willing to look for employment elsewhere even though his issues are similar to mine.
Issue
I've been gently nudging him and encouraging the idea that if he isn't happy here he should talk to the manager/bosses first and see if he can't come to a resolution. But since he's tried that they have been resistant and against any of the changes he and I proposed etc
So I've been actively encouraging him to seek alternative options. He is bright and I hate to see him being wasted here at a small start-up that really isn't going to grow much more (without drastic changes). But at the same time since I've started working there, he and I have become the top employees (in a small firm of less than 20, where half are essentially manual labor); and we do most of the other tasks (like data analysis, ordering, inventory management, other typical white collar tasks). If we both leave at the same time or near each other the firm will suffer greatly, there is no maybe or doubt.
Question
Now that he's actively looking (I helped him update his CV and gave him a few references in my network), was it ethical of me to actively encourage him to look for employment elsewhere, knowing that if we both leave we are essentially screwing over our current employer?
Note:
To explain more, he probably would have stayed there for the rest of his life if I hadn't been the one to encourage him to start looking.