I don't see why it couldn't be considered and aftershock even if it were on a different fault. Evidence the Big Bear aftershock of the Landers quake in 1992.

In that case the aftershock was 22 miles away and 3:08 hours later.

Although in today's case they're 44 miles away and 172 days apart.

As you pointed out, the coulomb stress map will be the key. Landers clearly showed a stress lobe in the Big Bear region, and the combination of these two clearly showed a lobe at Hector Mine although that's considered a "triggered quake".

At what point does an aftershock become a triggered event? And then a different event? Seems like it's up to fuzzy consensus.

Brian