I could have sworn it was on Tatooine, but I am on Tatooine and I can't find her anywhere. I have not been approached by her friend either. How do I do this side quest?

When you get off Ebon Hawk and are in the hanger on tatooine you are approached by a twilken and she will tell bastila and you about her mother being sick and that shes in the cantina. So you go out of the hanger and start walking when your approached by a Duro and he tells you about some really rude women in the cantina. So you go to the cantina and will find bastilas mother there.

make sure when you go to tatooine bastila in your party. I got this quest after finish the star map in tatooine (that's my first planet). after that I go to Kashyyk, met someone who tells me that helena(bastila's mother) is in tatooine.

back to tatooine again, then go to cantina to meet helena.

well tatooine was the last planet i wisited for the star map, and i already had my encounter with malak on my way there so bastila was "gone" so i couldnt meet her mother.

It's unfortunate, but that's essentially the quest killer -- once you lose Bastila all associated quests except the "RESCUE HER" sort tend to disappear.