"Anyway, they are a great couple."
Translation:Sono comunque una grande coppia.
I realize that it is often difficult to know when to use buon vs. bravo but I'd say that in this instance "un grande coppia" is also in this category of those phrases that must be memorized. I tried "coppia brava" and" coppia ottima" and these were marked incorrect, no doubt for good reason, but "grande never occurred to me.
We'll be sure to fix this, but here's a quirky difference to keep in mind: "grande" is one of those adjectives that changes in meaning depending on where you put it. "Una coppia grande" would be closer in meaning to "an old couple", whereas the most common placement for it to mean great would be after, "una grande coppia".