That soul will attain peace.
For a soul that has rendered bharanyasam, whether we perform or not, that soul will certainly attain peace without any doubt.
In general, the Tiruvadhyayan utsavam is done to proclaim that one who has passed away from this inauspicious existence will be auspicious at Srivaikuntham.
Because a sumangali would have had an auspicious existence here and will be auspicious at Srivaikuntham, a specific Tiruvadhyayana utsavam is not performed.