MP showing differential treatment - to whom to complain?

Dear all,
I have a very kind neighbour, Mrs X, whose child has been struggling with health issues for a while. This lady felt, understandably in my opinion, that she was being given the run-around by the NHS wrt their care, because the child has some congenital defects. She contacted CAB who told her to approach her MP. She did - and the MP told her s/he was unable to help, that the hospital "did not pay any heed to what s/he said" and "would not tolerate any interference" from them. So the MP declined to call the hospital or interfere.

A short while after this, Mrs X ran into another neighbour, Mrs Y, who mentioned that she had been having similar difficulties and that the MP had called up the CEO of the hospital the very same day, and that everything had been sorted within 24 hours. Further, when Mrs X went online and looked up the MP, she discovered that the MP and the CEO appeared to be very chummy - the MP was heaping lavish praise on the CEO publicly, and the two of them were soaping each other's back. So clearly they had the kind of relationship where the MP's word was heeded and paid attention to. It is blindingly obvious that the MP lied when s/he stated that they had no influence with the hospital.

The only difference I can see between Mrs X and Mrs Y is that Mrs X is black and an immigrant (although she has been here for over 30 years) while Mrs Y is white and English. I have no doubt at all that this is blatant racism. When I asked my husband what he thought, he laughed and said Mrs X and her child had the "wrong name".

I am outraged and disgusted by this behaviour. Mrs X is hugely distressed about the whole thing but wishes to complain about the MP's behaviour (as she jolly well should). To whom should she write? And what exactly will they do? Will they investigate properly (investigation will show that the MP rang the CEO for Mrs Y but not for Mrs X)? If investigation reveals (as it will) that the MP was less than helpful to Mrs X, while at the same time going out of his/her way for Mrs Y, what happens to the MP? Is s/he reprimanded and told not to do it again (rap on the knuckles) or will they actually face some serious consequences (as they should)? This MP was chosen to represent their ENTIRE constituency - not just the white people or the people they like.

So in my opinion they should face sanctions for their misconduct.

I don't suppose it's any use approaching Boris as he will probably make excuses for the MP like he did with Dominic Cummings.

Any advice re what Mrs X should do next would be hugely appreciated.

TIA.