They have contacted Delta customer services multiple times but seem to be told a different story every time they phone the States.
I don't work for Delta but, I used to trace baggage about 14 yrs ago!
No need to contact Delta in the US. Make sure your friends have filed a missing luggage report with Delta at Heathrow. They will be given a File Reference Number on WorldTracer as LHRDLXXXXX and should even be able to see what stage tracing is at online.
See
https://www.delta.com/baggage/landing.action
If the bag got misrouted to Reno - Then Reno should be able to see that the bag is needed at LHR by Delta and would Rush Tag the bag via first available flights to get to Heathrow and then LHRDL would normally have the bag sent to it's owner by courier.
This of course depends to a large degree on whether the original bag tag is still attached to the bag! If the tag goes missing then all bets are off. Unless the owner has say put name and address details on or inside the bag for example, it may never be traced.