Update: Episode is titled as Okaeri, Donna!
We still don’t know the title but here’s episode 19 description. It airs on 4/14/2009.
Donna and Kelly have an emotional reunion, but as their friendship continues to blossom, Kelly and Silver’s sisterly relationship takes a bad turn. Meanwhile, Dixon locates his biological mother and enlists Annie’s help to go visit her. Also, Naomi’s infatuation with Liam gradually increases.
source: 90210 Spoilers