"My girlfriend lives in Columbia, and she has a fiancee visa. Does she need to buy a ticket one-way or round trip?"
We actually receive that question quite often. If they do in fact have the fiancee visa, they'll only need a one-way ticket because they are coming to migrate to the United States and then apply for a lawful permanent resident status (a green card).
You would usually require a round trip ticket on a tourist visa, a temporary visa where you're just coming to visit the US and be a tourist, but if you're coming here on a K-1 fiancee visa, you have established the intent to marry within 90 days of entering the US and stay here to start a life together. So you definitely only need a one-way ticket.
We've even seen people write entire blog posts about how a round-trip ticket is required. It's just not true. It's absolutely never required for you to have a round trip ticket when a person has a fiance visa or a spousal visa. You may get people telling you that at travel agencies or even at the airline, and they're just uninformed. It's absolutely not true.
So don't believe what you're told. We do more of these than anybody in the United States - around 40,000 or so. You absolutely do not need a round trip ticket. Unfortunately, this is an internet rumor that just won't die.