Although GBH is a very serious matter, witness intimidation hold nearly the same weight. One can actually be prosecuted for W/I even if the GBH (or other offence) are let off.
First thing to do is contact the Officer in the case (OIC) For GBH, this should be a detective (CID).
Take it from there. Unless summonsed to court, one can not force the witness to give evidence. It is OK for me to say "Oh just go". But, I do not have any family and am as hard as nails. For this woman, perhaps things are different.
Saying that, I am all in favour of justice being served and believe ALL witnesses should attend court.
Contact the police, but go careful you are not colluding about the GBH case with the witness, because that could jeopadise the entire case. One must not speak about it!