I do not consider myself authoritative, and would not have personal experience (White, old guy), but my hunch would be they would have little concern.
Probably before 2010, I would say no issues, Poland spent a long time always on the other side of the wall, and welcomed about everyone from the West with open arms. Poland has changed, adopting a more Nationalistic approach, chafing with the EU, upset about migrant policy over the last few years. Being overwhelming Catholic, they are also conservative, causing concern among LGQBT travelers. I think these issues are more common to former Eastern Block countries than Western Europe.
While not a positive, there is some backlash against migrants, but that seems to be directed at more Muslim and Middle Eastern, rather than African. Also her friend will most likely be recognized as an American and a tourist, rather than a migrant. It sounds like her friend may be older than a teenager or college aged, that will help as well.
I do think urban areas less a concern than small towns or rural, but I do not think I would fear violence, or even outright harassment, but maybe indifference or exclusion.
There are some resources, blogs, and other forums that might more specifically address, but maybe this is a start.