Out of curiosity, I experimented with the Friends List earlier. What I found out is this:
1. Deleting someone from your Friends List does not notify the other person of his/her deletion as well as your activities.
2. Re-adding them in the Friends List will only show them that you logged in. You don't need confirmation from them to add them to your list once more.
In other words, deleting people from your Friends List is like selective privacy mode. They won't see what you're doing and they won't know that you removed or added them again.
The problem is this, if your character is part of a guild, it will still show in the guild chat window that you just entered the room. They'll still know you're online, but whatever you're doing, they have no idea.
I'm just sharing the result of my experiment. As always, it's best to have no people in your Friends List at all and your cheating characters to be guildless or have its own private guild that only you know about.