I think it might be theoretically possible for someone else to cast Teleport for a Garlean. At its core, the concept of teleportation involves breaking your body down to its component aether, hitching that aether to a ride on the Lifestream, and popping back out at specific preset aetheric "beacons" - in this case, aetheryte crystals.

The Garleans' inability to cast this spell is purely because they cannot innately manipulate aether, but their souls and bodies are still made out of it. If somebody else could break down the Garlean's body for them and send them into the Lifestream, they might be able to do the rest themself. As shown in-game, it is possible to cast Teleport for someone else.