Sorry, I don't know anyone in SA.
He can sue for defamation, but this is not DIY and a lawyer will be needed. The first step is a cease and desist notice and it can sometimes be worthwhile paying for the letter to be sent if there are flow-on effects the brother is trying to stop (eg with employment). Normally not worth the effort if it is just family and friends are being told. However I don't know the importance of personal reputation in some migrant communities which can place a higher value on personal reputation than others in the community.
Depending on the type of notices there may not be much he can do. If she is initiating multiple frivolous court actions he can have her declared vexatious meaning she has then has to convince a court she has a valid claim before they'll let her start a case.