If a left hand arpeggiated chord sits directly bellow a block chord in piano music, should the block chord be played when the first or last note of the arpeggiated chord is played?
It depends on the situation. Often the arpeggio feels like grace notes preceding the top note which is played on the beat. Sometimes the bottom note is harmonically more important, so it's the one that's on the beat, followed by the higher ones. Brahms sometimes writes downward arpeggios as grace notes leading to an on-the-beat melody note. Sometimes it's just kind of mushy around the beat somewhere.
In this example (the bar in the middle; treble clef both hands, right hand 8va?), the Ab is just harmonic filler, so play the bottom note on the beat.