I'm 90% sure there is a beautiful phrase out there to use to summarize this situation but since I'm a technical rather than artistic person I'm short of such creativity.
Here are some phrases that spring to mind but I think there are better ones waiting to be discovered:
- sleeping with the enemy
- guerilla tactic / guerilla recruitment
- if you can't beat them join them
What I'm trying to do is give a section title (for a booklet I'm writing) to describe this situation:
My favorite soccer team lost a winner-takes-all match against their fierce rivals. The coach of the rival team took a sabbatical (correction: he stepped down), and the rival team continued to win trophies. Meanwhile my team continued to struggle and eventually approached the ex-coach of their rivals to come out of sabbatical and coach my team. It worked out great as this mastermind coach won several trophies with my team too.