Hi, there.  There's no easy response to this, as it can be complicated to know what's depression and what is simply the loss of relationship or love for someone.  It's true that this colleague may be lifting her spirits somewhat and making her think that she's "in love," but it can very well be a distraction from her misery and that's why she seems to be so invested in it.  Is she being treated at all for the depression?  Has she tried light therapy?  People with seasonal affective disorder often benefit from spending time each day under a light box, which can be prescribed by a doctor.

Secondly, have you tried couples therapy at all?  This can be very helpful in getting everything out on the table in a safe environment.  Until we can communicate honestly with others, it's so easy to make assumptions about what someone else is feeling and thinking.  She may very well be going through a lot of internal changes, maybe as a result of depression or the inward focus it has given her and if you aren't aware of what's going on with her, and vice-versa, it becomes easier and easier to become distant.

I'm sorry you're having to go through this, as it sounds like you've been supportive of her through these depressions in the past.  But I hope she's getting help of some kind or that both of you can get some help together.  Only you can know if the relationship is worth salvaging for yourself.  I wish you the best and hope you'll let us know how things are going.