Why? Assuming that there isn't a non-compete signed by the former employee, the (former) employee has the right to use all their skills and knowledge to do whatever they want on the free market.
This wouldn't be an issue of non-compete, but of intellectual property - and I can't imagine there was no intellectual property agreement signed. I've never had a job that didn't have one...
General ideas (such as most of repl.it stuff) isn't covered by IP. I doubt that the (former) employee copied exact code in this case. It seems to me that he re-implemented some vaguely similar functionality. Moreover, most of that stuff existed before repl.it...