@youspacewalnut - This can be tricky. The way it's supposed to work if you drop one sim out is that they return home and should be waiting for you there when the other sim finishes the term and goes home themselves. But being that this sim had never even seen their new homeworld, it may not work as expected. I might have tried dropping both sims out. From where you are now on either save, can you drop you own sim out as well to see what happens, if that isn't what you had already done?
Since you used testingcheats to bring the BF into your household rather than a mod command, I take it that NRaas mods are not in play? It might have been safer to do this with a mod command and there are tools the mods provide to help find sims who have gone missing in between worlds if something like this should still happen.
