the General Medical Council is clear on this: 'You must not use your professional position to pursue a sexual or improper emotional relationship with a patient or someone close to them.' [http://www.gmc-uk.org/guidance/good_medical_practice/respect_patients.asp]
The British Medical Association have additional advice on this:
http://bma.org.uk/-/media/Files/PDFs/Practical%20advice%20at%20work/Ethics/Medical%20Ethics%20Today/updatechapter1june2013.pdf
Doctors must not drop a patient in order to pursue a relationship with them.
Some medics (psychiatrists, paediatricians) whose patients are considered vulnerable should not embark on a relationship with a patient.
However, if the patient decides to see a different doctor, it seems this breaks the professional obligations.
Of course if your friend is not a doctor he won't be regulated by the GMC anyway...