This is most likely the Druids "Hardened Skin" ability. It absorbs any physical damage with 5*Hero level. This is to compensate the Druids lack of "Warrior" secondary skills.
So in this case Solmyr is level 27, this means any creature/attack that deals less than 135 damage will deal 0
The neutrals on this map are very weak (20-40) and the hero already very good developed (month 2), so currently invincible.
I advise to increase strength of neutrals next game, my guess is he skipped the options, otherwise the neutrals would be much stronger.
It is also possible to right click on System Options Menu on Adventure Map and set new difficulty settings during the game! So he can, if he wants, increase the stack size if he thinks the game is to easy

Edit: I tested and if he sets difficulty to "Impossible" for stack size, the stacks will deal damage again ^^ (this step can be repeated several times during the game)

If this is not the source of the problem please report back