Get Your Dog Care Questions Answered by Experts
The answer in short is yes. The odds are low however. Your pup is very young and probably hasn't matured enough sexually one to produce sperm and two to even know what he is doing. Most male dogs don't mature sexually until 6 months of age or older.
I would have to say about 95% unlikely. You will know if she starts to look plumper or develops milk or you can take her in for an xray in about 30 days an ultrasound can be done sooner at about 15 days.
It is great that you have offered to pay but you have to remember that this wasn't all your fault. The owner should have informed you that she wasn't spayed and was in heat. The risks to her health are only during labor and delivery. Dogs do this all the time in the wild and survive just fine but she most likely will need help and need someone's presence during delivery. She may not have any idea what to do with the pups once they come so read up as much as you can.
Again, try to remain calm and remember that it is unlikely that she has become pregnant especially if she just came into heat since it is usually between 9-14 days into the heat cycle that they are ready to receive and able to be fertilized.