Is it the immobilizer that reads the chip in the key?

So, this is a friend's issue that I’m trying to get ahead on because I’m sure it’s going to require me actually fixing it, lol. So, I didn’t know all the exact details on the buy definitely the year make, and model are correct. So, he was changing the battery in the master fob and ended up losing the security key from what I can tell somewhere in the SUV because for several months the SUV drove fine but then it just wouldn’t crank one day and hasn’t since it has crank etc. I know it’s got to be that master fob ID chip key because the battery was changed in the fob once again, I’m thinking that unlinked that master key thus the resulting in it not working now. So that chip 1) I really don’t know what it looks like as I’ve gone all through the SUV looking for it myself. So, a locksmith came out and cut a new key basically a valet key nothing fancy still won’t start up. How do you reset that security function that part of the key itself that is read when you start up? How do you suggest I go about this?