I'm probably going to get down-voted for this but there are different scales of drink driving. For example, driving after a glass or wine or pint of beer would probably put you over the limit in Scotland, yet people do it all the time and nothing bad ever comes of it. Of course, it also depends on the individual's tolerance threshold - for some people, a glass of wine would put them on the floor and they obviously should not be driving until sober.

If your friend has had a significant amount to drink and it's obviously going to adversely affect his driving, then I think as his friend you have a duty to prevent him from driving that night and also to raise it with him and his family another day when he's totally sober. If I could prevent him from driving that night, I'd see no reason to call the police - doing that would very likely have a serious impact on his life. The objective here should be to initially prevent him (to prevent a serious impact on an innocent persons life), and then educate him so that he changes his ways. The police are only here to punish. Talk to him and involve his family - he will likely be shamed into changing his ways - and that's the end result you want, surely.