You can use #pokey with the pubkey of that user.
Discussion
This would give you notifications when that npub receives replies, reactions, or zaps. It would not notify you when that npub posts original content. At least, I don't see any way to make that happen in Pokey. @npub1v3tgrwwsv7c6xckyhm5dmluc05jxd4yeqhpxew87chn0kua0tjzqc6yvjh, am I correct in that assessment?
I am not aware of any clients that currently notify you when a specific npub makes a post.
Correct. At some point I'll implement notifications of own posts, and the case for being notified for user's post should do the trick.