OK, let's consider a couple of things. People are not necessarily right simply because they are fostering a dog.
The dvice you are getting from the foster is ridiculous (but don;t tell him that!).
Burns is an EXCELLENT dry food. You also really NEED to feed at least a dry food in part, as a soft diet can ruin their teeth and gums. So I'd definitely advise sticking with Burns as a great choice of dry. You also need to stop letting the DOG run the show. Please read this which has some good advice for dealing with what you are encountering:
http://board.cavaliertalk.com/showthread.php?t=24168
You can add in small amounts of cooked chicken (skin off) or trimmed ham (no fat) or other things, but only feeding them ham or chicken is NOT a good diet and the foster has not helped the situation of improving her diet by feeding her only this! I would not feed anything except the dry food until she is eating her entire bowl when it is put down for her. Wait a month or so, then introduce some goodies mixed in if you'd like. You need to get her eating a healthy quality dry food and not give her the opportunity to pick out just the goodies, which is what she is likely to do right now.
I might go back and talk to the rescue about your concern about this range of advice as I'd be going
ballistic if anyone fostering for me was 1) rude to the adopting family; 2) belittling the excellent food being offered by a company that is obviously supporting the rescue!; 3) feeding a foster so poorly. They'd be off my foster list in a nanosecond. I'd ask the rescue, and not the foster, for care advice anyway. I'd certainly let them know the foster was saying this about a sponsor's food.
Purina One is not a great food choice. I'd stick with Burns or go for James Wellbeloved or Royal Canin in the UK -- all are generally easily available at pet shops. Supermarket foods are all mediocre quality (that is why they are cheap and in the supermarket). Go for the largest available bags and simply close them tightly or get a sealed bin for the food to keep it fresh, and you will save money even over the cost of junky supermarket foods.
As noted on a different thread, I'd also asap order one of Ian Dunbar's books on dog care from amazon.co.uk as it will answer myriad questions you will have over the life of your new dog.