Hi mnb,
You are right to worry, with readings this high over a pre-long period will undoubtedly lead to ketoacidosis, which is very dangerous indeed.
Missing his lantus injection would raise his blood glucose the following day, maybe why he has been high all day. I think it would be best for your partner to see his doctor, or speak with his DSN who may offer some advice to keep his bg in single figures. There are courses for type 1's available, one is the DAFNE course, which helps teach people to dose adjust, in other words adjust their insulin to the amount of carbs they eat. It teaches folk to carb count, and as I have done the course I can highly reccomend it.
Good luck
Nigel