If the visa has been revoked then she should be told this by the authority that revoked the visa, not a representative of the airline. How did the embassy know she was travelling with Avianca? How did they know she was traveling at all? Had they contacted all airlines at all airports just in case she decided to travel? Surely if the visa has been revoked they would contact her to advise her of this. We know that embassies are not particularly bothered about our human emotions and feelings but to have people finding out verbally at the check in counter once they've purchased their tickets and made arrangements is a bit much. And isn't very professional.
It actually happens more than you think. You can search this board and find many cases.
The most common reason is too many trips and/or unusually long stays per trip. The US government doesn't foreigners using visitor visa as a way to have a second home in the US.
How frequent and long were her previous trips. Also, when you finally get a hold of her she will tell you what questions the POE asked her or informed her. That will give you an idea.
