Once someone becomes a 'breed fanatic' you can't talk any sense into them. Every individual in their favorite breed is perfect for anything, and if they don't win the Olympics at dressage, show jumping and reining, the judging is rigged!
Friesians don't actually do that well in dressage. There is quite a small number, 38 of them in the USA, who have HOY placings, and 95% of those placings are at the lower levels. Only 5 Friesians have HOY placings at FEI levels. The scores at the lower levels are indeed good, but the scores were gotten by only 32 Friesians in the United States.
Don't be fooled by the high set neck, thick mane and tail and lifted knees. It is VERY difficult to get these horses to be correct, to stretch, and to develop their back muscles, and get their hind legs under, instead of out behind themselves, no matter what 'type' of Friesian they are.
But what is 'doing well'? What do most riders really need? A top class dressage horse? No. In fact, most people would do far, far better riding something 'appropriate', and 'appropriate' is very often NOT a top class dressage prosect, but a horse with comfortable gaits, a laid back temperament, and not too difficult to ride.
There are a great many people who adore them because they are pretty, and I think like most 'breed devotees', they get very, very unrealistic ideas about the breed. People say the same for just about any breed they love - they're perfect, every single one of them can do anything.
But in fact there are problems. They don't move over the back properly, they are very hard to get a good canter out of, and they often have very serious problems with the heat. The hairy legs require a great deal of extra grooming or they get scratches, and that long thick mane and tail doesn't do well in our hot climate.
Friesians are a 'fad' now and we hear sellers proclaiming that every single one of them is perfect for the top levels of dressage. I think they look at them and think, 'Wow, look at those lifted knees, that horse will piaffe!' But the fact is, it does no good to just lift the knees, the hind quarters and back have to be working correctly.
So? To be honest, most people are far, far better off just buying whatever horse they like and they won't ever really be advancing or leaping up the levels in dressage anyway. They may as well have a horse that they think is pretty and that appeals to them.
For a person learning, a horse that is less sensitive, very tolerant of mistakes, and not too sensitive to bouncing legs and hands while the rider is learning - Friesians have a reputation for being very, very kind and tolerant.
Additionally, since most Friesians don't move over their back like a warmblood, they are far, far more comfortable to do a sitting trot or canter on (if you can get them to canter, see below). That's another thing that makes them ideal for someone learning, though not as ideal for excelling at dressage at the more difficult levels.
The only real serious problem is usually the canter. They can be very, very difficult to train to canter properly and I know quite a few professionals who have been completely stumped in their efforts to just simply get a young Friesian to canter at all. They simply were not developed to canter, or in fact, be ridden. They were developed to be fancy carriage horses, and carriage horses usually trot. The type of trot they have doesn't set them up well to pick up a canter, so it takes a lot of work to get the trot corrected and get a good canter out of them.
One of the most successful dressage riders to use Friesians, Sabine Schute-Kerry, wrote a very good article in one of the horse magazines about selecting a Frieisian for dressage. She gave some very good advice and while she loves the breed, she is realistic about what they can and can't do - something many people who adore them have a very, very hard time doing.
I honestly would not say a word to your friend. She should buy whatever horse she wants for dressage. Besides, if you tell her that the horse she adores isn't the best for dressage, you may be looking for a new best friend soon. People will do what they will do, it's best to just let 'em do it!