Your post Spec reminds me- that in a relationship there can be quirks that sometimes are part of general immaturity but not necessarily related to her mental health problems.
In this case you are mainly concerned with her lack of accountability. This can be a trait that is more associated with lack of development imo, so it could be something she will grow out of.
In term of dealing with any adverse behaviour there is of course counseling that I’d always recommend and there is trying some patience and wit with a “less is more” technique which is also using questions as a lever eg
She denies she is responsible for an event
”do you really believe you had nothing to do with that”
”but as you were the only one there...who else could be responsible?”
and so on. This makes the exchange engaging and she is left with solving the issues- sort of.
To get an idea along similar lines
beyondblue topic wit, the only answer to torment
also the following thread can help if you tend to argue
beyondblue topic relationship strife, the peace pipe
reply anytime if it helps