Larry,
Can I reverse if partner is a passed hand? What if there's competition? My partner says NO!
I say Yes!
A: Of course you can reverse opposite a passed hand!
Just because your partner passed, doesn’t mean you can’t have been dealt a big hand – enough to reverse.
As to “in competition,” – a reverse on your auction does show extras. If opener is minimumish, he should not reverse – even if the opponents come into the auction.
So, you will be happy to know you were correct on both counts!
Your partner should have a stopper in their suit to bid notrump.
Don’t expect your partner to be happy upon hearing all this – but it is 100% reliable information. -Larry