Can my husband pursue a defamation suit against his co-employee in the federal government?

My husband is a 26 years CBP employee with impeccable record. He has an existing EEO complaint against a vindictive boss who falsely accused him of utilizing his government vehicle for personal purposes. He was fully cleared at the time he was being vetted for promotion. Due to IA investigation, he was not promoted even though fully cleared of all allegations. Last week same employee (99% sure it is her) filed IA complaint that he is "skimming" marijuana issued to his K-9 unit and "selling it" and bringing it home for us to smoke. There is zero evidence of this and he will be cleared of all charges. But, he is again up for promotion. To date, they have taken his gun, barred him from his office and his government vehicle and have him reporting to another supervisor in another location known to all employees as "the bad boy chair". He wants to pursue a defamation and slander suit against this employee as she has accused him of criminal behavior and damaged his reputation twice, this being the final straw. We are fully aware of cost issues.