On FC servers, item loss is never refunded (there’s no reliable way to track this, and it would clog the system). You simply have to use your own methods to verify and vouch for the members you invite to the guild. By inviting them to the guild, you gave away any materials they had access to.

On other servers, there is possibly a means of getting justice in this situation, but you will need to refer to their rules and such for details.