Salma Hayek -- Halle Berry's Expert Witness in Custody Fight

Halle Berry has an ace up her sleeve in her child custody war ... her good friend Salma Hayek ... and Halle believes her sexy friend could help her convince a judge that moving to France is necessary for her and Nahla.

TMZ has learned ... Halle's lawyers just filed a declaration from Salma ... in which the actress says she knows firsthand that France is a much safer place for a celebrity because of tougher paparazzi laws.

In the declaration, Salma -- who's married to a French billionaire -- tells the court ... she spends a ton of time in France with her young daughter Valentina ... and doesn't have to deal with the kind of paparazzi crushes she gets when she's in the United States.

As we previously reported, Halle wants to move across the pond with her daughter and fiance to escape the constant paparazzi attention and recent intruders at her L.A. home.

Case in point -- Salma and Halle went to dinner together in Hollywood last night ... and walked right into a paparazzi flash storm as they exited the restaurant.

12:50 PM PT -- Court update: they're still doing opening statements. The judge just broke for lunch.

10:00 AM PT -- It's on. Halle just arrived at court. Gabriel is there too.