Delta Airlines From Heathrow

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