Well, the "correct" way to do this is to ask Expedia to split the PNR and then just rebook the one pax to a different return.

Not sure if that is easily doable via Expedia.... for sure, not a problem if you would have booked with LH directly.

The only disadvantage is, if you run into irrops it might be they route the travel party on separate ways. But I have been quite successful to always sweet-talk the agent handling the rebookings to put all pax of a group on the same new flights - even on different PNRs.

Then there is "the other way". Your third pax "misses" his return flight. Just be a no-show. If it happens once, nothing will happen. But the problem is, you also have to buy a separate ticket to get back to the US. Which will be quite expensive if you go with a one-way flight. You might want to book another return trip and also miss the "return" part of that. But I would book it on another airline, e.g. SAS, Air France or KLM and not via Expedia.