I have an intact male and my gal-dog went through two heats before I spay her. People were constantly asking if I were going to breed them. No, we were not!
So yes, your neighbor might end up with a litter but they might be good at managing their pack. The trainer of our IPO club had intact males and females and no accidental litters.
Hope, its really not your responsibility to check on their dogs. There is little you can do in the end even if you do go outside frequently. The most you can do is let your neighbor know that they will mate if not separated and produce more pups. But you said you were 14 right? They might just not listen to you, simply because people don't like taking advice from younger people. (Actually, some people have trouble taking advice from anyone to be honest..)
If they don't change their ways or spay/neuter the female or male then it will likely happen again. But that's up to them to be aware of.