It is unlikely that a court would consider 30-year-old infidelity to be a contributing factor in a divorce. Once infidelity is known to a spouse, if the spouse chooses to forgive the infidelity and remain married, it is much diminished as a factor in a divorce which occurs years later. To the extent that the infidelity continues, or a new relationship precipitated the marital break-down, a court would likely be more interested in the infidelity as a consideration when dividing marital assets. Your brother would benefit from getting specific guidance on this from a divorce attorney in his state.