morbid studio
welcome to HRS
if you can posket the expense of some treatment that may be your best hope- most apartments have some soundboard in the "partywall" which is often 2 walls back to back with the SB in between....older apartments, you're lucky if there's insulation.
As RawD mentioned, bass will be the biggest peace breaker, and basstraps would go a long way in helping this- I' trap the ceiling to back wall, bot corners and place your monitors facing away from your neighbor
bass traps on the back corners would also be a help as would some diffusers- these can be built/bought so you can take with you
good luck
with any luck, your neighbor will be a musician!