Just keep going over and visiting if you have the chance before Christmas. Bring her maybe a biscuit everytime you go and maybe if you go over YOU let her out and do a few of the things you'll be doing while the owner is gone. Then at least she won't be shocked when the owner leaves. There's really not much you can do till she gets used to you. Mercedes is like this with strange people she just needs longer to warm up to them.
Hope this helped a bit.