I'm a 26yr old t1 diabetic, and I've recently got engaged so I guess I'm as good as anyone to give you advice!!
Well, life expectency is the same as anyone else I guess. If you don't look after yourself you are asking for trouble, but then some people look after themselves perfectly and get run over by a bus! This is only as big an issue as you make it. If your girlfriend looks after herself and is sensible about this (but of course, not being too strict as this I think will lead to unhappiness) then she is doing the best she can. If you read back to some other threads, I posted one about a website that records stories of other diabetics - there is a story about one lady who was diagnosed as t1 in the 1920's and is still going strong! There are plenty of scare stories around too, but the best we can do is try to be optimistic about all this.
As for a cure - don't hold your breath. Presume there isn't going to be one soon. There really isn't anything on the horizon that I know of. There is the pancreas transplant, but then the person would have to take anti-rejection drugs for the rest of their life (which, funnily enough, can cause diabetes).
There's the insulin inhaler, but this still doesn't seem able to give an accurate dose. We all just have to keep looking after ourselves the best we can.
Now, insulin - I am on the insulin pen and to be honest I take about 5 shots a day (sometimes less, sometimes more). If you've read through some of the previous threads you will realise that one treatment that works for one person may not necessarily be the right one for someone else - you and your partner would be best of speaking to a professional about treatment regime.
about the dietry sacrifices you will have to make - following a diabetic 'diet' is basically a healthy eating plan for anyone. It will probably do you good to eat like this but a diabetic can eat almost anything as long as it's in moderation and is done sensibly. It's a question of balance (i.e. eating a small portion of cake or ice-cream after a meal, with maybe a little extra insulin).
Sorry for the long essay here! I understand this must be at the very least worring for you, if not frightening, but don't let it get the better of you. We all need to face our own mortality at some point, and as I diabetic I've had to face it sooner than a lot of people I know. But with modern day treatments there's no reason why your g/f shouldn't live a long and happy life. I hope this reassures you!