If the neighbour was knocking because their music was too loud then no they don't deserve any refund. @Nancy1846
obviously they must have spoken to the neighbour if he said he asked them to turn the music down.
do you have anything in your listing about quiet hours? It's a difficult one as it's a new neighbour so you don't know if he was being over sensitive or they were being unreasonable.
a neighbour asking them to turn down their music would not 'ruin their trip'
I wouldn't be looking to offer any type of refund in these circumstances.