Whether it's right or wrong depends on whether you want to accept an offer from someone who clearly is not in the position to offer, namely has sold his house STC. The estate agents are acting in your own best interests in doing so. There is no point accepting an offer from somone in such a position. What happens if the estate agents then take the house off the market and your firend cannot sell his ? It means you have lost out on potential buyers in a position to offer.
Yes they must tell you of an offer if one is made - but they are wasting your time and theirs in accepting such an offer.
I tell my estate agent that I never want to even know about viewings unless they are in a position to buy.