If the matter is as you have described, it is permissible for the guard to offer Zhuhr Prayer instead of Jumu`ah, (Part No. 8; Page No. 193) so that he may be able to guard the station. This is for the general meaning of the juristic proofs indicating that it is permissible to leave Jumu`ah under that excuse.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.