While firing, EV2's damage is not affected by buffs or the loss of buff.

If an EV2 starts the firing the beam, and a monk next to her uses heroic wave, her damage is not altered.

If the monk uses heroic wave, and then EV2 starts to fire, they damage is indeed increased, however, it'll stay increased while she's firing even long after the buff ran out.

It's not just the monk, It seems that this happens with the mystic as well.