I wish this guy the best, but a "technology solution" is not going to be able to address the psychological burden of RSD, nor the executive functioning issues of ADHD. An Airtable sheet with a "Mark as Contacted" flag is something a business analyst would come up with after speaking to an person with ADHD.
RSD can't be solved 100%. At best, you can give yourself some guidance on how to resume contact with someone:
1) When was the last time I was in touch with this person?
2) How would I rate my relationship with them on a scale of 1-10 -- this is important because in adulthood friendships can fade even if nobody did anything wrong
3) Why do I feel they would not want to hear from me?
4) List 3 light topics of conversation that are low-risk, and choose one to reach out with. That can help reduce the internal resistance, which is a key factor in RSD.
RSD can't be solved 100%. At best, you can give yourself some guidance on how to resume contact with someone:
1) When was the last time I was in touch with this person?
2) How would I rate my relationship with them on a scale of 1-10 -- this is important because in adulthood friendships can fade even if nobody did anything wrong
3) Why do I feel they would not want to hear from me?
4) List 3 light topics of conversation that are low-risk, and choose one to reach out with. That can help reduce the internal resistance, which is a key factor in RSD.