I am a 25-year-old woman and have been underweight since I was 3 years old. I have a small frame and look fairly healthy at my low weight, but still consider myself too thin. I think I'd look my best in the 'ideal' BMI range, and I'm only 5 lbs away from that now!
I'm about a year into this way of life, and I'm maintaining at 11 to 6 lbs heavier than my previous setpoint (I was between 95-100 lbs for about 10 years) without having to stuff myself, for t he first time ever. A lot of it is muscle, since I'm pretty lean I can see my new muscles clearly! I look 10x better IMO. I'm aiming for 5-20 more lbs, but I'm not in a big hurry. I do still have to put in some effort to overeat/have a schedule if I want to actively gain, and I hate doing it, so tend to keep those periods short and then try to maintain what I've put on until I can face the thought of counting calories and eating more than I prefer (it makes me nauseous).
Eating this way has also cured a ton of health issues for me, which is the reason I started.
I would recommend 'primal' rather than strict paleo for hardgainers. Plenty of carbs - by that I mean between 100-200 grams mostly from the most harmless sources like tubers and white rice, depending on activity. There is no reason for anyone with a normal metabolism to be scared of starch, and that goes double for those of us with super-efficient metabolisms.
You can tell your friend that I also do hardcore yoga (lots of weight bearing) and weight train, with free weights and lifting as heavy as I have the equipment for right now. Weight-bearing exercise is pretty important for me if I want to gain. And it's essential for almost every woman's health to build more lean mass.