In the previous issue of Sudarshanam, Svami clarified adiyen’s query that even if it is a Perumal prasadam, a Srivaishnava must not accept that prasadam when offered by a non-Srivaishnava; it is considered a dosham. On that day, without being aware of this, aidyen had already partaken Opiliappan prasadam (sarkkarai pongal) and Tiruppati laddu prasadam. Would adiyen have accrued effects of sin because of this? If so, what is the atonement?

There are types of doshams that can occur through food; first occurs when one partakes a food that is not Perumal prasadam; the second occurs when we eat food given by others, which will be considered anacharam.
In this instance, this devotee has partaken Perumal prasadam. This will not result in the first dosham; If he is an orthodox person, there is an impact on his orthodoxy (Anacharam). There are certain atonements for this like partaking Panchagavyam. Or if the bhagavata generally has a habit of eating food prepared externally by others, no further harm can happen.
Notes:
It is better to look at the message at a higher level and understand the essence. Avoid overthinking by ascribing perspectives and asking queries based on these.