Yes it does disclose a problem although your friend may have a different scientific question in mind (although it is hard to guess what that might be). Forecaster 3 who guesses 105, 110, 120 does even better I think you will find.

There are a host of options. In the medical field an influential paper for the case where you are comparing two measurements is

@ARTICLE{bland86,
  author = {Bland, J M and Altman, D G},
  title = {Statistical methods for measuring agreement between two methods
          of clinical measurement},
  year = 1986,
  journal = {Lancet},
  volume = 327,
  pages = {307--310},
  number = 8476
}

You might also want to look at mean squared error https://en.wikipedia.org/wiki/Mean_squared_error

Yes it does disclose a problem although your friend may have a different scientific question in mind (although it is hard to guess what that might be). Forecaster 3 who guesses 105, 110, 120 does even better I think you will find.

There are a host of options. In the medical field an influential paper for the case where you are comparing two measurements is by Bland and Altman available here entitled "Statistical methods for measuring agreement between two methods of clinical measurement". In the thirty years since publication it has been cited more than 38000 times according to Google.

You might also want to look at mean squared error https://en.wikipedia.org/wiki/Mean_squared_error