I am a doctor. 50 units a week is likely to cause damage to his health at some point, even if does not cause any immediate problems.
If he was admitted to the hospital where I work and told staff he drank around 50 units a week, every week, we would be seeking to highlight this and discuss our concerns with the patient. He would also be monitored for signs of alcohol withdrawal and started on B vitamins (not saying your DP is alcohol dependent, merely that those drinking at this level are at greater risk of being so and we monitor this carefully and treat prophylactically to avoid the risk of serious complications such as DTs and Wernicke's).
Yes, there are some people who can drink to excess for a protracted period of time with little in the way of ill effects- they are the exception rather than the rule though. I have seen first hand the terrible effects of liver disease and neurological damage caused by alcohol- it's not a risk I'd be willing to run.
As to whether YABU to do nothing, I would say that depends on whether you think his alcohol consumption is problematic in any way now or has the potential to become so. It also depends on the importance you place on his health- yes, he might be one of the lucky ones who doesn't end up with health problems- how great a gamble are you happy with?
I would also say the fact that he doesn't want to talk about it does ring some alarm bells for me. This suggests he knows it's excessive, but is avoiding confronting it head on- surely if he was perfectly sure that his drinking was fine, he would just say so? But again, you know him best- perhaps he reacts to this when any questions are raised.
All of that said, you can't make him change his drinking habits, regardless of whether he is dependant on alcohol or not. All you can do is explain your concerns and ask him to take them into consideration.