Leo's can be a bit pompous and a bit vain. But that's mostly harmless. Mostly they want to be appreciated for their efforts and achievements. Which is hardly a crime.
Ex : If a Leo gets into an Ivy League, or a cushy job. You may find them always saying went to XYZ Undergrad or Grad school. Which though pompous is not unjustified, as the Leo has worked their *** off for it.
The Leo friend I have, do brag about his business at times, but he has sort of earned that right to brag - he had given his everything into it. And yes at times he sounds boring, but he has very few friends with whom he can share and brag a bit.
But how arrogant Leo will be depends on his/her moon and mercury as well. If they have Aries moon/mars, Leo mercury, they might come across as more arrogant and abrasive.
Most Leos deep down just want to hear how awesome they are, particularly from people they care for. If you have a Leo friend or partner or relative just tell them now and then - You rock/You're a brick!!
That said, immature people exist in all signs and if this woman is too immature and crazy to be with, she's bad news . Ditch her.