Hi,
The concept 'mixed conditional' refers to making hypothesis about how something that actually happened or didn't happen in the past would affect the present, so you need different time references in the main clause and the condition. That's why I think your first example is not ok, it should be: What would you do if your girlfriend had told you she had (got) a new boyfriend. (present time if-past time, time not tense). The other one meets this requirement and, to my mind, it is ok. (not a teacher, though).
Other examples,
If I had studied medicine (past time), I would work in a hospital (present time)
different from
If I had studied medicine (past time), I wouldn't have met my wife (past time).
Hope this helps you.
Regards.