OK this is weird. If you try to book a one way on 10/11/20 on DL, it shows the flight as available, etc.
The Support Desk (I'm on with the friend and DL) says it's not available to sell as inventory as part of the round trip ticket.
The DL agent is going to DL.com to see and send the screenshot I guess to the Support Desk. Wow. Never seen/heard this before.