If you said "I missed you so much!" to a person a natural response would be "So did I!" It means that the person you are greeting missed you equally while he/she was away. "So do I" has a similar meaning, though it is not proper grammar. Is your friend a native speaker? Did he/she just accidentally use the incorrect verb tense?
yes he is a native speaker,he lives and was raised in US. So,if someday someone tell me that he misses me so much it's better to say "Did I" than "Do I" right ? And in this context "So" means = And ?
If I get it " So did I" = "me too" ? So if we say "I'm hungry" an the other person respond "So do I" is it correct?