When two developers are stuck with a problem and someone else wants to help them, he needs to first make up for the previous discussions the two have had and which he missed. I believe the “just” questions are a good way of doing this.
“Why can’t you just X?”
“That will brake Y”
“I see, so how about XX?”
“That will conflict with the requirements, see here”
“Ok I see where you’re at, I can try and aid you with some more brainpower now”