Hello!
It´s so nice to see people interested in phrases such as this! But as far as I know, you can say "hakkını helal etsin" only for living people, as a deceased person "hakkını helal edemez".
In this context "hak" has a meaning like the injustice or wrongs done agaisnt someone. And when they "hakkını helal etmek", they forgive the wrongdoings against them. Obviously this requires the person to be alive ^^
On the other hand, saying "Hakkımı helal ediyorum." about the recently deceased is a common practice here. The belief is that when you forgive the wrongdoings the recently deceased may have done against you, they won´t be punished (or punished more lightly) for those sins in the hereafter.
This phrase also has a response phrase, "Helal olsun".
Person A: Senden izinsiz kitabını almıştım, hakkını helal et. (I took your book without asking for permission, forgive me.)
Person B: Ne hakkı canım, tabii ki helal olsun! ("Hak", what "hak"? Of course I forgive you!)
Sorry for the wonky translation there, it´s a bit hard to translate this as there´s no close phrase to it in English.
So, helal olsun! 
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