Is this multiplayer? Are you both playing the same version (1.0.28 or 1.1.0-Beta2)?

If one of you has *script* mods (such as the grenade mod) installed then you'll have different checksums. Uninstall all of these mods.

Lotpacks, texture mods and sound mods do NOT affect the checksum.

Current correct checksums for the currently live versions are:
1.0.28-428q for version 1.0.28 (this is the default branch)
1.1.0-Beta2-zx7b for the 1.1.0 beta.
You can check by looking in the top right corner of the main menu screen.

Issue was resolved, I dropped into the server that they were playing on