Just did exactly this last month. I suggest she get the eMed rapid tests and use them for both the predeparture and return to the US tests. It's cheaper and easier than the SFO rapid test, and you get your results before you show up at the airport, so if comes back positive it's a lot easier to rearrange things.

ExpressTest at LHR during the layover was also very easy, though not the cheapest option.

Overall it was super easy any nobody but the check in agent at SFO looked at any documentation.