Here's the sentence in question:
We had been on the train for eight hours, but it felt like eighty due to the cramped conditions.
I felt this was a compound sentence. A colleague, however, said that if the second clause were to be isolated, it would not make any sense because the word "hours" has been elided. In order for it to be a genuine compound sentence, she insists it should read:
We had been on the train for eight hours, but it felt like eighty hours due to the cramped conditions.
Can anyone clarify this for me, please? Incidentally, if it does fail the compound sentence test, is it a complex sentence by default?