Look, amjon is saying that it's not possible for a competent ultrasound technician who is actually looking for a twin at 26-weeks gestation to miss it. People are countering that by citing examples of where hidden twins were missed when health care providers weren't really looking for twins. Me personally, I wouldn't be as sure as amjon is; stranger things have happened in the world of medicine than a competent ultrasound technician missing a hidden twin. My own advice to the OP's friend would be "it's extremely unlikely, so you should not get your hopes up too high, but anything is possible."
Or in other words, you can disagree with amjon without being disagreeable. Try it sometime.
Not surprised to see wristwatch24 here though, giving "thanks" to every person who gets in amjon's face. WWJD... Whom Would Jesus Dogpile? Right, wristwatch?