What should a person do who advised a girl to pray two rak'ahs as a prayer of gratitude to Allah during an illicit relationship, and then they both repented, but then it became clear to him that such a prayer is invalid, and he fears contacting her to correct the error out of fear of falling into the forbidden and opening the door to temptation?

It is not permissible for a Muslim to be in a relationship with a foreign woman, and this person has done well by repenting. However, dedicating two rak'ahs of prayer daily as an expression of gratitude to Allah has no basis and is an innovation (bid'ah). What is obligatory upon him is to delete the woman's email address as a preventive measure against temptation.