Does mental illness excuse wrong behavior?

Recently, a friend of mine apologized for making hurtful and unfair comments to me. (It's not the first time she's done that.) She said that she's been struggling with depression, and she's now on anti-depressants and in therapy. I'm not sure how to take that. I feel for her, yet I also feel like I'm being manipulated into overlooking her bad behavior because she's "sick." How should struggles with mental illness figure into explanations and apologies for wrong behavior -- if at all?