Just adding the link. http://www.santamonicataxi.com/ - also my #1 choice.

Express taxi is not bad either http://www.passengerswanted.net/

Reponse times here in Santa Monica are usually pretty good - even if you call needing one right away.

Here is a link to the authorized taxi companues for LA:
http://www.taxicabsla.org/

In LA, I usually use United Independent, but I always call well in advance. The few times I have called needing a cab right away, they have taken 45 minutes. Called other companies too - but they also had a long wait. Those times have been in the afternoon, so that might just be a busier time.

I think she should be fine (quick response, if not a can waiting) at major attractions. The relative's house or if she wants to leave a restaurant would be the longer waits.