He wouldn't be. AetherSX2 is based on LGPL-era PCSX2 code and license changes don't apply retroactively like that (otherwise, imagine if SDL changed its license to GPL and instantly forced half of PC games to go open source).
It would only prevent him from using code added after the switch to GPL, which is not a problem because AetherSX2 ended development long before PCSX2 switched to the GPL.
It would only prevent him from using code added after the switch to GPL, which is not a problem because AetherSX2 ended development long before PCSX2 switched to the GPL.