A much loved German coworker of mine is leaving on Friday. What are some nice things I can write on a card for her?
I am really bad at writing cards but I can translate things.
PS: Don’t worry if few people answer during the next few hours. It 3:40 AM here. 
Herzlichen Glueckwunsch means heartfelt goodwishes. ‘Herzlichen Glueckwunsch zum Geburtstag’ is heartfelt goodwishes on your birthday. Is she leaving for a new job? Maybe ‘Herzlichen Glueckwunsch mit seinem neuem Job!’ (yes, job is Job in German).
I’m rusty with genders. If somebody could double check, it would be great!
How about “Herzlichen Glueckwunsch mit Ihrem neuen Job”, since the OP is addressing the departing co-worker directly, and not in the third person?
You’re right. See, I told you I was rusty.
you might want to change that to “Deinem neuen Job”, which is less formal. This depends on the relationship at work of course. If it was your boss or another superior, you would use “Ihrem” in Germany. If it was a direct colleague, it would be “Deinem”. I would also add: “und alles Gute für die Zukunft wünscht Ihnen/Dir HDS” (The same distinction again between formal/less formal)
That means: “and best wishes for the future from HDS”
So, in total it would be:
“Herzlichen Glückwunsch mit Ihrem/Deinem neuen Job und alles Gute für die Zukunft wüncsht Ihnen/Dir HDS”
vielen Dank!
She’s been very nice about my mangled German. This will be wonderful to write on her card.
gern’ getan.
keine Ursache.
take your pick. 
They both mean “You’re welcome”.