Using a borrowed rocket is not a problem in terms of rules, what I said the other day mostly applies to blueprints you repost. However you're supposed to do the challenge with a rocket you built yourself indeed. Sorry but I can't accept it.
Please be careful when using that word because it's a serious accusation. Not mentionning the author is one thing, but stealing would be claiming a rocket he didn't make.
Dumrocketblueprints never did that so please don't call him a thief.