Here's what I found on this topic, which is in tune to what I was told by my dr -
"During pregnancy, your basal metabolic rate (BMR, or the number of calories you use each day) will increase, and you'll need more calories to support the extra work needed for fetal development. During the first trimester, most women usually don't need to increase their usual daily intake of calories (i.e., 1,800 to 2,000 calories) unless they need to compensate for starting a pregnancy underweight. But even if extra calories aren't consumed in the first trimester, you should still make balanced nutrition part of your daily wellness plan.
During the second and third trimesters, you will need an extra 200 to 300 calories each day. But that doesn't mean feeling uncomfortably full: for example, just one piece of toast and a banana can supply those extra calories.
There are exceptions to the extra-calories rule: women who start out under- or overweight, women who are very physically active, and women with certain medical conditions should talk to their doctor about specific caloric needs."
I think the problem the midwife had with your friend saying she was eating for two is that it's a philosophy that nowadays they try very hard to counteract b/c it was an incorrect belief a few generations ago. However, if your friend only put on a stone and she's 22 weeks, it sounds like a healthy weight gain to me, so I'm not sure why the midwife even mentioned the weight? Your friend may have said it tongue in cheek but the midwife didn't take it that way obviously!
Personally I've not been eating for two, I've been eating the same as always but I'm throwing in a couple extra pieces of fruit a day. I'm still running for an hour at least 3 or 4 times a week but it's much, much tougher then before pregnancy (I'm 17+ weeks). Even with all that, it still seems the weight goes on much easier now during pregnancy. It's for sure important to keep weight gain under control, not so much for asthetics during or after pregnancy but mostly b/c the risks of problems with blood pressure, gestational diabetes, etc go up with rapid weight gain. Having said that, it doesn't sound like your friend is putting on anything but healthy weight so I hope she ignores this midwife's attitude.