Oh no. What a predicament! I hope the owner returns to get her dog...Do you know here well? Does she seem like the "type" or person to do something like that? Were they planning on moving? (Like house up for sale?) Or do they rent and had to be out by the end of the month? Maybe she moved to a place where she couldn't have 2 dogs? Not that that is an excuse! Perhaps you can find out more tonight from another neighbor or the landlord/real estate agt about where she has gone. It can't be legal to just abandon a dog! Did you leave a note or anything in case she comes home today and sees the dog tied up at your house. She may see him there and think you want to keep him...?
Good luck! I'll keep my fingers crossed that maybe she had to leave w/o him and is worried sick!