The only method I have personally found to be reliable for grilling/pan-frying chicken breasts to a relatively uniform doneness is to pound them very, very thin with a mallet or rolling pin. Thin, as in scaloppine-thin, so that it cooks almost instantly in the pan.
Every other stovetop-only method is almost certainly going to produce a bland, tough cut, regardless of whether you press it down or not. As noted in my comment, my usual (lazier) method that does not involve pounding is to get a nice sear in the pan, then jam in a temperature probe and bake it in the oven until it's done (the USDA recommends 165° F, I usually don't go quite that high).
If pressing the meat actually accomplishes anything at all, it would most likely be to just squeeze out whatever tiny amount of precious juices the breast does have, and possibly give you slightly more even cooking on the exterior only; it will not help to cook the interior much faster unless, as stated above, the cut has been pounded extremely thin and flat, at which point it doesn't really matter.
P.S. Salt and olive oil is a terrible "marinade" for any cut of meat, especially a chicken breast. The salt is just going to get suspended in the oil and never reach the meat at all, and the oil itself won't have much of an effect on such a lean cut. You really need to change your marinade as well, preferably to something water-based (or at least not 100% oil).
Short answer - you are right on all counts and she's wrong. Tell her that, she'll love it. ;)
The longer answer is that boiling a frozen piece of meat, especially one that is thick in the middle like chicken breast is exactly the opposite of what you want to do as you'll cook the outside but the inside will still be frozen, and boiling (as you rightly point out) will remove flavor from what is already IMO flavorless to begin with. Any cooking method will dry the chicken out whether boiling, baking, or frying, so by cooking twice you are liable to end up with food that is overcooked on the outside and undercooked on the inside.
Cooking frozen chicken isn't a great idea from a safety and quality perspective but it can be done if the meat is cut thin, so your point about butterflying it is very valid. What I do personally is I thaw chicken meat (thighs in my case) partially in the microwave, then I slice it into 1/2" (about 1.3cm) pieces across the grain before cooking in a saute/stir fry of some sort. I like partially rather than fully thawing in the microwave because it gives a better taste/texture than a full thaw in the microwave, and it is very easy to slice when partially frozen.
Best Answer
Yes, this technique will give you approximately the same results as Nando's. You want to use an oil with a relatively high burning point like sunflower, rapeseed or groundnut - olive oil may burn.
Another point to remember is that Nando's chicken breasts all have the skin on: this adds flavour and helps keep the meat moist. For the best results, both in terms of not giving yourself food poisoning and avoiding dry chicken, use a quick-read digital thermometer to ensure the breast is only cooked as much as it needs to be, but at a safe internal temperature. It should reach 75ºC in the thickest part.