I have a Metabox frontend submission form in frontend. All the posts submitted from this form are in "draft" status.
I'm looking for a solution for a moderator to validate these posts.
I have tried with the Metabox user dashboard but the posts displayed are his own, not those of the people who submitted the posts from the frontend form.
First, try to modify WP_Query arguments with rwmb_frontend_dashboard_query_args (check docs); e.g., adding this would let the moderator see all posts:
$args['author'] = 0;
You will have to use conditions within above-mentioned filter to allow only selected users/roles see all posts in a dashboard.
Next, not sure what you mean by "moderation", is it to hide/publish the post? If so, I use custom field group with radio buttons having options corresponding with post status, e.g., "publish", "draft". The field group is then assigned to a post type. Upon submitting the form, you might fire wp_update_post to update post status according to the option selected in the form's field group with rwmb_after_save_post action (see docs).