Singapore Airlines flights arrive into the G wing of the International terminal at SFO.
The Departure level is located directly above the arrivals level. After going through immigration and collecting bags, your friend will pass through customs who will direct her to the exit on the right-hand side (the other side is for connecting flights).
After following the path (it snakes around a little) they will exit into the public area of the terminal. The exits will be directly in front of them, but they should NOT go through them as that's the wrong level.
Instead, turn right, walk maybe 20 metres/yards, and they will come to an escalator going up (slight off to the right, but it's very obvious). Once at the top of the escalator they will be on the departure level - loop around to the left and the exit will be very obvious. There's also an information desk at the top of the escalator (just off to the left) who will be happy to help.
If they'd prefer to avoid the escalator, a few metres/yards past the escalators on the left are the elevators/lifts - just select Up and then the Departures floor (the same elevators also go down to the parking garages)
When booking the Uber they'll need to select what door they are at - International Departures Door 5 will be the nearest if they follow these instructions.