Call the Huntington in San Marino to see if their early wisteria on the edge of the rose garden/ Japanese garden is in bloom. Descanso in La Canada has a pretty one in the rose garden but it blooms much later.

Is she looking to take pictures? The Huntington doesn't allow wedding photography but Descanso has a lovely space in the rose garden for weddings.

There is a huge--as in ginormous--wisteria in Sierra Madre (not too far from Diamond Bar). How big? So big that the Guinness Book of Records has classified it as the World's Largest Blossoming Plant. (Go figure.) The city has built a festival around what they call the Lavender Lady. Last year the festival was on March 25. This year it's on St. Patrick's Day, March 17, but it's always held sometime in the late middle/early end of March. (Link to further info on the festival is attached.)
I'd suggest that your friend might want to zero in on a date in mid to late March, but microclimate is important. It might be worth a call to a large a reputable nursery in Diamond Bar (not a big box store) and ask when their wisteria will be in bloom.
Kay

Here is a link that might be useful: Sierra Madre Wisteria Festival