If there is atleast one thing good about the meal, compliment that first.
Make the necessary correction by asking questions like
* What spice did you use? seems it affected the taste
* Is there still salt? it's like the food needs more salt
* When did you pour the red oil?
jus5 ask questions pertaining to the wrong and correct it.
If you have eaten any previously nice meal prepared by the person,you can tell her that today's meal is not like the person's handwork who made the other one previously,I believe she would ask you how,then you can point out the problem with the food.
Or you can by commending her efforts by spending time to cook the food but then she should try to improve on the cooking to make it tastier next time.
As always, its best not to negatively criticize the meal/cooking out-rightly without suggesting ways to improve it.
I would try to be around when next the person intends to cook the same meal again, then I would actively tell the cook why putting a certain amount of a particular spice/ingredient at a particular time will make the meal taste better!
Maybe if you can offer some suggestions or tell them you had something similar but done in a different way, they might see things in a different way.
Before you go to your comment about their por cooking, try to point out some positives. Like the effort they put into the dish or something. Then you go to the comment and don't forget to give some recommendations for their next try.
Don't forget to give some encouraging words before you end your comment.