If you put a flat (matte) colour on top of blonde it will go khaki green so don't do that BUT she could put a warm dark brown, like a chocolate brown on top and the warmth in the colour will stop it going green
Also if her hair is orange then the warmth in the orange will stop a matte colour turning it green also HOWEVER if her roots have taken to the blonde then that WILL go green. When someone is going from having their hair dark to blonde, we use a colour stripper that ALWAYS lifts it to a bright, firey orange which believe it or not is what we want then we put the colour on top but this really shouldn't be attempted at home.
If you insist on doing it at home, go into salon service and buy NUMBER 4 DIACOLOUR RICHESSE (which is brown) and 4.15 DIACOLOUR RICHESSE (which is chocolate brown) and buy the developer to go with it. Mix half and half of each of the 2 colours and add developer. This will turn out pretty dark but at least it will NOT be orange and it will have a nice, rich tone to it.
I'd recommend you go to a salon and have it done but as I described above, that's what I'd do if you came into the salon to see me.
When people go from dark to blonde, it always goes slightly orange, you have to persevere with it, keep having highlights on top to eventually get rid of the chemically treated hair underneath which is a bit of a nightmare.
The other option with your hair being so short is that if you insist on being blonde then a professional may be able to keep stripping it to bring most of the orange out then put an Ash tonner on it to try and cancel out the warm (orange) tone.
Hope it all works out for you.
xx