If (as is the case) our friend has had the misfortune to meet some unpleasant/curt people rest assured their attitude is thankfully not endemic.
Sometime Brits have a reputation of being less approachable. Kinda like the person at a party who once you go over and talk to her/him you find out they're a really nice person.
People are not "rude to you because you are an American".
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