It seems to me that you have three choices:
1) Have her briefly tell what happened.
2) Have her narrate what happened, in detail.
3) Do it as a flashback.
As the other posters have said, which you choose depends on how important the incident was to the character.
If you choose option 1) (have her briefly describe) the advantages are that it's quick and easy, the information gets related with the minimum of fuss (and having a character 'tell' has more impact than having the author 'tell', although both are less powerful than 'showing'). The disadvantage is that there will probably be no emotional impact to it all.
Option 2 is a difficult one to handle, as in real conversation people never usually go into that level of detail, i.e. we never really speak in a prose-like way. So it might be difficult to get it to sound natural. The advantage would be that the incident would be related in real time (i.e. no flashback), and the more detail you gave, the more emotional impact you could give it.
The third option is the flashback. The disadvantage of this, as with all flashbacks, is that it halts the story in its tracks and is somewhat jarring for the reader. But once you were there (in that past time) the incident could be related with all the detail and emotional impact you wanted.
As with every decision in writing, there are advantages and disadvantages to each choice, and your job as the writer is to decide between them - what you specifically want, and what price you're willing to pay.
As already said, it depends on how pivotal the incident was.
good luck with it.