Some designers get bent out of shape if you try to sell the finished product from their pattern (whether they have the law on their side or not). Some will be less than happy if they feel your results aren’t “good enough” or up to their standards.
Others will be more than happy to give you permission
If she is planning on selling in person not online, they won’t know though and since it’s not against the law in the US, it’s not like she’s “cheating the system”
Yes. In the US at least you can sell the finished object without having to ask permission. Even though some designers insist that you have to ask or have to give them credit. Neither is required by law here in the USA