Well Erin, Madrid is amazing all year round so I would have her over whenever she can make it.
Obviously in December you ladies will not be able to enjoy the outdoor dining on the "Terrazas" that the warmer months offer but there is much more to the city than that as you know
As for the climate, I am from N.Y. so I do not find the winters here in Madrid bad at all. In fact, my girlfriend and I spent the month of December last year in N.Y. and she (being from Madrid) thought it was the coldest place on earth
I guess being from California, it will be different for you and your friends but I encourage you to take advantage of X-mas time in Madrid for it is, although different than the summers, a great time to be here with all the holiday cheer and festivities
You can see my Madrid calendar at
www.multimadrid.com/madrid_calendar/mad_calendar.htm for events that take place in the city throughout the yeat to see what the winter months are like.
Also, the -2�, -4� temps are not the norm for the winter here in the city so you can assume it will normally be a bit warmer (ie 1�-5�) which is not bad at all. Keep in mind that the cold in Madrid is not as bad as people may think because it is a "dry cold" rather than a "wet cold" (with high humidity) that is usually more bone-chilling.
Saludos,
jer...
p.d. If you want her to see the Madrid you know and love (tunos, terrazas,etc...) I would highly recommend her waiting for May for it is, in my opinion, the best month to be in Madrid, once again, go to
www.multimadrid.com/madrid_calendar/mad_calendar.htm and flip through the months until you hit May, you will see what i mean

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