Just because you think she was born in a country that drinks a lot, doesn't mean she was raised around drinking. There's plenty of binge drinking in America, but I was raised in a home with zero alcohol, and none of my friends' parents had alcohol in the house either. If my parents ever drank it was like one glass of wine a month after the kids had gone to bed. If she comes from a home that's anything like mine then, "3-4 times a week now with old colleagues and does come back rather drunk" 100% sounds like a drinking problem. If that was how my husband behaved I would be pushing him into rehab.
All that being said, if your husband's drinking has never negatively impacted your marriage, then it might not be a problem. But your son's drinking is clearly an alcohol problem as it's negatively impacting his marriage. And seeing as how he's behaving the same as the example your husband set, your husband's drinking is now her problem.
To think that London, or anywhere else for that matter, does not belong to any one demographic