It’s from the frame of reference of the probe you make fuel calculations. The various observers don’t necessarily need to agree on the ordering of events (Relativity of Simultaneity), but they will always agree on the laws of physics, which are the same everywhere. If it takes a given amount of fuel to accelerate a mass to a given degree, everyone will agree on that point. It might take som calculation to make that clear to all of the observers, but they will agree regardless if some are in accelerating reference frames, and others inertial.