Will this fail inspection - breaker brand mismatch?

Inspectors absolutely do catch wrong-brand breakers, sometimes. Depends on the inspector and how thorough they're feeling. alot of them will just glance at the panel and check the workmanship, but the ones who open the door and look at the breakers know exactly what doesn't belong in there, and a mismatched breaker brand is one of the first things they spot becuase the logos are right there staring at them. The real answer is on the label inside the panel door. Every panel has a list of breaker types that are listed for use in that exact enclosure. If Siemens is on that list, you're fine and can stop worrying. If it's not, then even though the thing snaps in and physically works, it's not a listed installation and that's a legit fail, not a nitpick. GE panels historically don't take Siemens, and a Siemens breaker saying it 'fits' usally means it physically seats in the bus, not that it's actually approved for it. That's how people end up in this spot. The good news is this is the easiest possible fix. Kill the main, pull the two wires off the breaker, snap it out, and snap the correct GE breaker in. Ten minutes of work once the power's off, and the breaker is maybe twenty bucks at any big box store. Way better than rolling the dice on Thursday and having him flag it, because then you're doing the same swap anyway plus scheduling a reinspection.

So before you lose any more sleep over it, just open the panel door and read the label. Match what's printed there against what's in your hand. If it's listed, done. If it's not, swap it and put the Siemens one back in its packaging for a return, since most stores will still take it if it's unused.

Did the same swap last year before my final, only headache was the return. The big box store by me wouldn't take the breaker back once the package was opened, so I ate the cost on that one. Call ahead or check thier policy before you crack the packaging again. Also snap a photo of the label inside the door with your phone before you head to the store, saved me a second trip when I couldn't remember the exact wording they wanted. My inspector never even touched the panel door, just looked at everything with it open, but I slept better with the matching one in there.

Kind of a side question but does it matter that your electrician friend was involved at all? When I did my subpanel the inspector asked who did the work and if I said a licensed guy helped I feel like he looked closer, not less. I just told him I did it myself and he walked me through a couple things instead of failing me. Maybe being upfront helps your odds too, worst case he tells you to fix the breaker and comes back.

Seconding the swap idea with a story of my own. I did a breaker change the night before an inspection once and the thing that saved me was a $15 non-contact voltage tester. I waved it over everything before touching a screw, and honestly it calmed my nerves more than flipping the main off did, because I knew for sure the bus was dead and not just probly dead. Also, when the inspector showed up he spent most of his time tugging on my ground wire and checking torque on the lugs, not staring at brand logos. So while you've got the panel open anyway, give everything a firm tug and snug anything loose with a screwdriver.