I think your friend made a good move to marry the fiance' but in my own opinion, it would be useless to send the K-1 back, since they would treat the K-3 as a new case. Remember, this time, they would instead start working on the immigrant petition (I-130) and once approved can then apply for the K-3...much more paper works than K-1.
But the fact that marriage would be a stronger foundation to prove their relationship is already a good evidence.
In my case, I had a K-1 denied (with ex bf), the year 2003. Immigration officer said no evidence of relationship and they returned the papers to USCIS. Perhaps, it was true because after my ex bf knew what happened, he abandoned me and the papers (he's a jerk! and I was stupid), and never cared for me at all....I'm now thanking the immigration officer for that!
I then met another man (now my husband) who was so sincere, kind and understanding, came to visit me in the phils 3 times before we got married. He went with me during my interview at the US embassy in Manila, armed with complete evidences. (2 inches thick), that were not even bothered to look at..hah! They just did a quick look on my husband's taxes and payslips. I got approved, and now happily living with my husband here in the US
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