In this situation, I would keep them in the coalition for as long as possible. That way, you can focus on the real enemy, whilst you also have bonus VPs for the coalition. As well as this, it means your enemies will be wasting resources - they’ll have to fight you and the inactive.
In contrast, if you kick the inactive and invade them, that’s wasting time AND you’re forced to fight your real enemies as well as wasting troops on your coalition teammate.
Added to all this, there’s always a possibility of your teammate coming back!