Be polite, be cool and be truthful. Women know when you are lying and (lets face it), anyone reading /. comes off as patronising at the best of times, so its best to downplay that as much as possible.
Unless you are a specialist in the field and analyse raw data, your state of knowledge on this specialist topic is actually formed by a combination of an appeal to authority and Occams razor. Scientific method is a great way to avoid falling into error, but knitting your socks anew every morning is a very inefficient way to keep your feet warm.
You believe that at least some of the primary source specialists in this area are applying scientific method and believe that the secondary and tertiary commentators, which is usually what you have read, can be trusted to accurately report the findings and critique the methods of the primary sources. An explanation which assumes that all commentators are collaborating in a non-scientific is vastly more complex than an explanation that the sources are actually applying the methods claimed and reporting findings and analysis accurately, so you believe them.
Your girlfriend is using the same techniques. She trusts her grandfather as an authority in the field and considers any explanation in which he was duped or misled to be far more complex than alternative explanations. She may not be aware of the history of alternative explanations; but you never met her grandfather.
She probably isn't expecting that you accept her grandfather's account uncritically or without reservation; just that you don't dismiss it out of hand in an arrogant way. So listen to her explanation uncritically. Ask relevant questions such as where the site was and what was her grandfather's involvement etc. You can say things like "Wow, if that's right it would be amazing". Ask if she would be interested in finding out more about it. If not, don't push it. If yes, then help her find out stuff. You will find out more about her from either a lack of curiousity in finding out more about it, or the lengths that she is prepared to go to by rationalising data to avoid disrespecting her grandfather than her initial account of his story. Some people find the latter trait very attractive in prospective mates, others find it infuriating.