Hi all, hope you're all doing safe and well. I'm currently stewing over my morals and what would be the best course of action for this. I tried to keep things as vague as possible in case any of my coworkers use this subreddit.

Two weeks ago, my boss [40F] reached out to me via phone call and told me that a position in my department is opening up (the previous person in this position retired) and that she wanted to give me a heads-up in advance before the job was posted. She proceeded to say that because of my qualifications, and current performance, she thought I'd be a great fit for the position and encouraged me to apply to the listing. She said that I'd have her recommendation for the spot. The way our department works (as I'm sure many others) is that internal candidates have priority for applications, and because of her recommendation, unless a recall candidate or the previous person were to re-apply, I'd probably be a shoo-in for it.

I currently share a position with my co-worker [20M]. He has worked in this position since August 2019, and I've been in my current role since January 2020. I feel kind of bad because he trained me and has been here longer, and we make a great team, and it's so sudden so I'd feel like I'm blindsiding him. Our position is also getting busier because of the Covid situation, and the job isn't really a 1-person fit anymore. However, I'm really interested in the new position as it's more along the lines of where I want to be moving forward/upwards in terms of career, and pays about 10k/year more. I guess my question is, should I give my coworker a heads-up that I'll be applying to the position? One of my best friends says that I should leave it up to my boss to handle/let him find out on his own terms, so he doesn't feel pressured to respond some sort of way in front of me/process all his feelings about it in front of me. I don't want to "go behind his back and apply" though, you know? I guess I don't really know, haha. He does know that the position was posted (HR let us know during the staff meeting as per protocol), but not that I'm interested in applying/our boss recommended me.

Any thoughts or suggestions would be so greatly appreciated!

TL;DR: Boss recommended me for a higher position in the department. My co-worker who I share my current position/role with knows about the job opening but doesn't know that she recommended me. I'm thinking about applying/taking the position, but am wondering if I should give him a heads up so that I'm not blindsiding him as our current role is quite busy and not really a 1-person fit anymore.