I say, split them up and try different approaches with different batches of tadpoles.
Tadpoles survive outdoors on their own quite well; it's how Nature intended. Here in downtown Houston, eggs are laid in puddles at the curb, which dry up, get wet again, dry up, and eventually the eggs hatch and the tadpoles turn into frogs and toads.
If you could recreate conditions as close to nature as possible, they should have a chance. An above ground pond sounds OK, if you can heat it somewhat. The thing is, since it's above ground, it does not have the insulating capacity of a natural in-ground pond. That's why you'd need the heater.
Can you section off part of your pond, so the fish can't get to one corner?
You could add LOTS of underwater plants, and hope it provides enough cover for the tadpoles.
Summary of options:
1. above ground pond, w/heater if possible
2. sectioned-off portion of your existing pond
3. in your existing pond w/underwater plants
4. in your friends' pond