How can my friend act regarding what she conveyed to her brother about his wife, and the ensuing quarrel between the brother and his sister, given her fear of the dispute escalating to family division?

The sister must exonerate her sister from the accusations leveled against her and strive to reconcile the spouses. It is forbidden to corrupt the relationship between them. There is no need to inform him of anything that might lead to him leaving his wife, even if it necessitates lying or insinuation.