Can the hero who goes last push a monster off the end of the momentum track?
That is correct. See page 15 of the rules under 'Getting Wounded'.

If its momentum card is not on the edge of the momentum track when wounded, it shifts a number of steps equal to the number of remaining hits; if it reaches the edge, ignore the remainder.

Tactically, if there is only 1 target left, and it is not next to the Edge, then the last player should strongly consider taking the Plan action after powering and slotting Action cards.

Your advise was sound.