I recently did the Trans-Siberian Railway in May. Siberia in May is surprisingly hot, often over 30°C, but in Mongolia at night it can drop well below freezing. I found with a load of the usual t-shirts, about 3 thin fleece jumpers and thin rainproof jacket (mac/anorak type thing) I was fine to cope with the cold. And then I had jumpers for intermediate conditions, and they all fold away quite small. Fleeces also dry quickly when washed - very useful when backpacking.
Also look into base layers, which pack up very small and do a lot to keep you warm.
If you're not going to regions like Nepal and Northern India, don't even think to take warm clothing with you. That's from my experience. I'm travelling for a three months in Asia and I arrived here from Russia, and the only place you can possibly get cold is heavily air-conditioned transport. You may feel quite cold in sleeper buses and trains in Thailand. But all what you will need in this case is a thin fleece shirt or blanket. One time I saw some guy used his sleeping bag as a blanket -- it's really clever option for light backpacking too.
I'd recommend you to go to the airport with your friend or relative. In airport you can change the clothing and give all the winter stuff to your buddy to get it back in town.