Cassandra has currently been mentioned only once in Halo's storyline. A letter written by her and addressed to John-117 appeared in Conversations from the Universe. Its text has been reproduced below.
As you might imagine - though I know you won't -
I've heard quite a bit of your recent adventures on
the Halo. I'm glad you made it; I have to assume you
always will. Recovery is going well and I am proceeding
on to fourth-stage rehabilitation. It is significantly
more painful than previous stages, but I'm happy to be
skinned once again.
I hear that you will be testing the new Mark VI.
I will definitely look for the reports on how it works
out. The specs are obviously improving, but the choice to
incorporate further Covenant technology somehow makes
my skin crawl.
We miss you, John. I've asked Sergeant Johnson to let
me know if you're ever near the M25L Recovery Station
and perhaps I'll be able to come see you. I'm hoping I'll
have the chance soon. Today, humanity feels pale and
thin with only ghosts to defend her heart. I feel much
↑Regarding the source that has been paired with this footnote: though Bungie does not explicitly state that Cassandra is an ex-Spartan, their response, when asked about the matter, very strongly implies that she indeed was a Spartan.
↑Cassandra's letter to John-117 was written while she was residing at the facility in question. She references the Battle of Installation 04 and refers to them as "recent," indicating that she was alive and at M25L for a period of time after the battle. However, the exact date of the letter is unknown; Cassandra could have been relocated or discharged from the facility at any point after the letter was written. As such, she was residing at M25L, but no sources indicate that she is still residing at M25L.