I've interviewed a lot in the last 12 month and companies reactions afterwards differ widely, some come back within a week, some never do. Yes it is rude but in my view also an expression of how they treat people. Unless they are just over worked.
Your friend could call them and politely ask about the status. If she has an offer and feels confident she might even mention that, as in: Look I'm in demand, you are my preference, but I need to make a decision now. Or you don't mention it at all and quietly take the other job and see what happens. Ususally there is probation period for both sides within which the notice period is just one week, so she could still jump ship quietly.
Best of luck