When she looked up she was delighted.
I agree, the comma is needed.
I'd put a comma in too.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Anyway, I'm not surprised that many people would use a comma to separate the two clauses, but saying it is "needed" in that short sentence is perhaps overstating it a bit. But puting a comma there is not wrong.
As in other things, people can vary in how often they use commas.
In my opinion, the sentence doesn't suffer any from lack of a comma. The two clauses naturally separate from each other, even though you don't really need to pause between the clauses. (When A happened B happened.)
That's my opinion, and so far I'm only outvoted 5 to 1.