The answer might depend on when/how the "early returner" is travelling. If buying a completely sepparate ticket that's one thing. If, on the other had they are using the return half of their ticket as (a probably very small part payment) of their new ticket then that will cause the booking and ticket to be separated from the rest so no problem when everyone else checks in.
Now for a bit of history...... My wife and I booked London/Dallas/El Paso then Phoenix/San Francisco followed by San Francisco/London. All in the days of paper tickets. My plans changed. She flew the London/Dallas/El Paso alone. We went together Phoenix/San Francisco. She went San Francisco/London alone.We never told BA (or AA or UA) and nobody batted an eyelid!