Comment by FussyZeus

My go to for this is to say "I'm guessing $solution didn't work." Because then you're supposing a solution that most likely the other person did already, and you're communicating that you'd assume they'd already thought of it, and in the odd event that they didn't, they can just say "Oh, I guess I didn't think of that."

That said, whenever I walk into a scenario where someone's been plugging away at a problem for a good length of time, especially when it's someone I trust and know to be competent, almost invariably the answer is yes, they did try it, but asking in this way often then illicit the person to explain what went wrong. And after a few rounds of it we've eliminated a number of the more obvious solutions that didn't work, and I'm now up to pace and can try and provide better ones.