Short answer: Don't.
Long answer: There is no reason why you should need to totally isolate yourself from the people who care about you in order to ctb. Most reliable methods that people actually use are effective reasonably quickly, to the point that it isn't necessary to totally cut people out of your life in order to find the unsupervised time and space to reliably attempt.
Moreover, it sounds like you're still in the planning phase, intending to ctb, but not fully committed yet. Ask some of the users who've been around on the site for a long time: ctb is hard. People get scared, survival instinct kicks in. Your first attempt can fail, you can fail multiple times. You can get so close and then something can change and you may eventually decide that this isn't something you really want after all. It happens quite frequently. And if it does, if you're not 100% fully committed and absolutely sure you're gonna go through with it and get it done, you don't want to end up in a position where you're totally isolated and don't have anyone to rely on or turn to. That's a horrible place to be.
There are two potential reasons I can think of for why you'd want to do this in the first place. The first is to make it a little easier on yourself, tie up loose ends, so to speak. And again, while it's true that isolating yourself may make it a bit easier to ultimately go through with, again, it's a horrible place to be mentally and emotionally, on top of the horrible place you're already at if you're seriously planning an attempt. If this really is what you need, you'll find a way to make it happen with or without people in your life (many people do), so I don't think that's ultimately a good cause to cut yourself off.
The second potential reason I can think of is to spare this person pain. Maybe it seems like, if you pull away from everyone who ever seemed to care about you, it won't affect them as badly if/when you go. But it doesn't work like that. The people who knew you or had any connection to you are going to be traumatized either way. That's just an unavoidable reality of ctb: it hurts the people around you. I'm not stating this as a reason not to do it: if you really need a way out, your own needs come first. But I am saying that if you go through with it, this friend of yours is going to be horrified regardless. The difference is she can be horrified that she was unable to stop a friend she cared about from taking that leap, or she can be horrified about that and blaming herself out of the belief that maybe she could have stopped you if she'd just held on a little longer or harder when you pulled away. Any form of tricking her into letting you go just twists the knife, and makes what is already going to be an awful, traumatic experience for someone who seems to care about you so much worse.
And lastly, from the way you seem to describe it, this seems to be a person who really does care. I know you're planning to ctb, and you at least sound like you're serious in your intent. But right now, you're still here, with someone in your life who seems to care and maybe wants to make it a little more bearable. Human connection is really the only thing we have at the end of the day, once you strip everything else away. It's the source of most of the only things that are actually worth existing for. And even if you're seriously planning to step out, there's no good reason to cut yourself off from one of the only worthwhile things you still have. Why not let your last days be just a little less lonely?
Not sure if I've stepped out of line on this particular occasion, but this is truly how I feel on the matter. In any case, wishing you the best.
