This actually took me a few tries to get right. It isn’t exactly difficult, but to be non lethal you have to get things exactly right to be able to finish the mission. This guide explains how to eliminate Lade Boyle non-lethally and without being detected.
There is a visual version of this mission at the end of this guide.
There is a duel which is an optional part of the mission in the garden. Simply use a sleeping dart crossbow instead of a pistol to win to pass it non-lethally. I have an ability that stops time which made the duel very easy indeed.
First I suggest you loot the upper area of the Boyle Estate. There is a bunch of mission clues and back story to read along with a lot of good loot. The area is fairly simple to navigate without detection. There is a bedroom at the end of the main corridor:
Which contains a secret area. When you enter look to the right and use the switch:
When you do a hatch will open near the ceiling. There is some loot up top.
Also there is a basement vault, I have written a separate guide for finding and opening the vault which contains some consumables and a bunch of loot.
I don’t think you have to search the place for information as long as you know the right target. The easiest source of information is Miss White which is a lady with a fly mask:
She asks you to get a drink for her and once you do she gives you information about what the different versions of Lady Boyle are wearing which should give you conclusive information about which one is Waverley (the one in the white). To complete this mission non-lethally you have to talk to this guy with a sack on his head:
He offers to take Waverley far away, which counts as eliminated. He asks you to meet him in the basement, which is down the stairs from the room which contains the food, with the unconscious Waverley. Next you have to speak the Lady Waverley Boyle:
1. Let me get you a drink?
2. I can save your life if you’ll listen.
3. Someone wants you dead, but I can offer you a way out.
4. You can choose either one, I found asking her to follow me the easiest.
Take her down to the cellar, choke her out and give her to the main with the sack on his head.