Use has been.
Perfect tenses always refer to "completed" actions, or actions that we are "done with" in some way.
What does it mean for "be absent" to be "completed"? Sometimes it's easy to understand what's meant by "completed" and other times it's more about the speaker/writer no longer considering it something to pay attention to or deal with.
Let's try was ...
I hadn't seen her, she was absent yesterday.
Here, we expect her to return soon. We can't use "in a while" because that's an expression that is used to express that you don't expect something to happen immediately anymore.
With has been ...
I hadn't seen her in a while, she has been/was absent for almost a week now!
we are implying something like that you have:
given up on expecting her to return,
given up on expecting her to do something she was supposed to do within the time she hasn't been around,
given up on expecting her to be absent according to normal patterns,
or something similar.