Had trouble understanding specifically as to what you are asking.

Are you referring to how you can do a post and then under Options choose to "forward sats to" another SN user? If so, that's not using their Lightning address, it's just an internal ledger transfer.

But I suspect it just shows up in notifications for them as if they had done the post themselves. I'm not sure though.

Or are you asking how, if someone receives a tip to their account on SN and that tip was sent using their Lightning address, how does that appear? There is a notification that sats have been deposited. There is no "memo" / "message", and SN can't know if that was a tip or you depositing that yourself.