I would skip Olvera Street. It is a throwback to the bygone days when Hispanics were a minority in L.A. Nowadays, Hispanic culture in L.A. is... everywhere. Like its next door neighbor Chinatown, Olvera Street is in seedy downtown L.A. where nobody goes unless they work there or have jury duty. Also like Chinatown, you will find far better ethnic cuisine of your choice in whatever neighborhood you happen to be in. If you really need a genuine Mexican serape, find a weekend flea market at a community college or visit the Tijuana border. I would only recommend Olvera Street to someone changing Amtrak trains across the street at Union Station and who has some time to kill.

Oops. You did mention your friend is arriving by train. So you will already be there. Park at Union Station and walk over.