The second mole doesn't get time to attack. Instead, it lets out a squeal of pain and Theseus may feel some heat on his back before the mole is forced to dive back into the earth.

Newt has his wand out; hand shaking slightly. He had set the creature on fire. By no means, did he think it would kill the beast, but it was more to distract it from attacking Theseus. And it was at least successful, because the mole has returned back underground; using the earth to quickly put itself out. At the very least, this gives Theseus a bit of time to handle the mole before him before the other mole tries to attack again.

That is...if the second mole is still intent on targeting Theseus.