wrote:

I have sort of a weird situation where I'll be flying out of
Heathrow to LAX at the end of the month on American Airlines.
A family member has to change her flight so that she also flies
out of Heathrow back to the States, but she's on Delta. (She
was supposed to fly out of Rome back to the west coast at the
end of the month, but she's changed her mind and wants to come
back sooner.) So, will Delta offer service on another carrier
for part of the trip, or am I just plain out of luck and will
have to purchase a one-way ticket? Joy

Delta do not fly from Heathrow - they use Gatwick.

If at www.delta.com you go to reservations and feed LHR into
the system then LHR will not be recognised - you will be prompted
to accept LGW.

For London - LAX on Delta the routes shown all go Gatwick to
Atlanta and some have another intermediate stop in addition.

I suggest you double-check your relatives flights before
making any changes to your own.

(Ten or so years ago I flew on a Virgin Atlantic plane from LHR
on a Delta ticket, but that codeshare has long been discontinued)