I hope you are doing well, insha’Allah. Thank you for the question.
Tawba from backbiting is a process. It entails:
Scholars differ as to whether the backbiter must seek pardon from the victim. Some opine that it is not stipulated as informing him might cause greater distress and anguish. The expiation would then be for the backbiter to praise the person in the very same gatherings in which he backbit him. In addition, the backbiter should stop others from backbiting the victim to the best of his ability. [Tafsir Ibn-Kathir]
According to the Shafi’i school of thought, if the backbiting did not reach the ears of the person, it would suffice him to be remorseful and seek forgiveness for the person as per the detail provided above.
However, if the backbiting has reached the person, then ibra (exoneration) will not be valid unless the details are disclosed to the person. This is because a person may forgive certain things to exclusion of others. [Mughni al-Muhtaj, Nihaya al-Muhtaj]
Backbiting a Non-Muslim
As for backbiting against a non-Muslim, Shabralmillisi has the following to say, “The more correct view is that he (the backbiter) prays a prayer of forgiveness for him other than forgiveness of shirk (associating partners unto Allah) or for an increase in his wealth, etc. whilst expressing remorse.” (Hashiya al-Shabramillisi) Moreover, as per your suggestion in the question, the best prayer for a non-Muslim is that of guidance.
Furthermore, a collective prayer for those whom we backbit would not suffice.
The expiation requirements for backbiting are telling of its severity. It should serve as a deterrent from this major sin. The devastating impacts of backbiting on the familial and social fabric of our Muslim community cannot be overstated. May Allah grant us the tawfiq (knowledge and practice) to follow through on His commandments and desist from His prohibitions.