Yes, that would be illegal and a breach of the fiduciary duties owed t the condo community.
You certainly will take a risk by informing them or making any statement concerning such if you ultimately can't prove it - you risk a per se defamation action against you.
Attorney
19 years legal practitioner: real estate, collections, estate, civil, business, and criminal law
You could or should contact the board of directors for the HOA - it's actually their action because that's from whom he's stealing from.