If your friend is willing to take the proper precautions then I see nothing wrong with her experimenting.
In this case I would not let the birds naturally breed, due to the extreme size difference.
As long as she takes proper care of her birds and takes whatever steps necessary to prevent injury, then let her experiment.
You never know, it just might become a breed.
Granted the Orpingtons and Silkies each have many many years behind developing them, someone thought it was worthwhile experimenting to create them. Just look at the Jersey Giants or some of the other American breeds, someone had a flock of Java chickens and decided to throw in some other breeds to create all new breeds.
I don't think we should stop experimenting just because we "have enough breeds as it is."
I breed Java and Dorking, but there have been many many many times I have considered cross-breeding(not between those breeds, but other breeds) experiments. I have a book of breeding experiments that I would like to try someday and then some experiments I like to just mentally play with, like a breed created from Onagadori and Silkie.
We are not talking about mass production of mutt chickens, but a backyard keeper of chickens wanting to put a few birds together "just to see what we get".
There are many sources on genetics online that your friend can read up on to get an "idea" of how the offspring will turn out.
-Kim
