Hiring lawyer is the only way out

Without knowing the circumstances, nobody here can tell you what excuse your friend can use to back out. Commercial real estate is "buyer be aware", no consumer protection like residential real estate.

Lawyer will find some bullshit reason to get some money back, mostly some kind of "fraud" committed by the seller. This requires the lawyers comb through every piece of paper from this sale, until they find some "misinformation" made by the seller, and then they'll magnify this minor issue into a "big fraud", it will cost some money. But the cost is minimal compare to the amount involved, potentially $3 million loss.