I think it is great to have a partner who does 70 mile rides with you but if she really does want to ride faster, then you have to work out why she can't ride faster than that now.
What kind of terrain is that 12 mph achieved on?
Since I got unfit and put a lot of weight on, my average speed on my forum rides has dropped to about 8 mph! I used to do more like 12-13 mph which I consider reasonable for rides including 6,000-7,000 of vicious Pennine hills. Even at my fittest, I was only doing about 15-16 mph on long rides in these hills. So, if you live somewhere as hilly as this, I'd say that her 12 mph is actually already pretty good.
If you live somewhere flat, then, okay, 12 mph is pretty slow, but not annoyingly so. It would just take a long time to do long rides. She certainly wouldn't be riding particularly far in daylight in the winter.
If her speed is okay on the flat, but it is hills that cause her to slow down, then her power-to-weight ratio is the problem. She would have to lose weight, increase her power or both. If she isn't slim, then losing weight would be the obvious way to get faster uphill and doing 70 mile rides (including some hills) at 12 mph is bound to help with that.