I have bought and sold several houses, and from what I have seen, this is entirely legal. In fact, it is commonplace and generally advised for a buyer to have a home inspection done after the contract is signed. The buyer must pay for the inspection, and arrange with the seller or seller's agent for a convenient time for the inspector to come. Nothing prevents an inspection from being done "for free" by a friend or relative, and it sounds like the buyer did coordinate the visit properly. Often a contract is contingent upon a satisfactory inspection result. If this buyer is going to use his stepfather's observations in that capacity, the stepfather should have the credentials to support this, and should submit a formal report of findings. Barring that, any observations made by the stepfather have no more weight than those made by the buyer himself.
If there are actual issues with the house, any buyer should be aware of them. That is just truth in advertising. In most places, sellers are required to declare any known issues (leaks, flooding, termites, buried oil tanks, etc.) to prospective buyers. If a buyer's home inspection reveals an issue of which the seller was unaware, after the inspection it must be included in that list for subsequent buyers.
Bottom line: whether this visit can impact the sale will depend strongly on the wording of your contract. If the buyer can get out of it only if a qualified inspector identifies actual problems, that puts constraints on his stepfather's observations. Results of even a formal inspection can be disputed, though that may require you to pay for your own inspection. Undisclosed problems with the house must be addressed, though, that is only fair.