So, since you've not specified which part of the UK you're in, I'll go with what I know.
Sexual Assault under terms of S. 3 SO(S)A 2009 states that:-
"If a person (“A”)—
(a)without another person (“B”) consenting, and
(b)without any reasonable belief that B consents, does any of the things mentioned in subsection (2), then A commits an offence, to be known as the offence of sexual assault.
MODES OF PERPETRATION (2)Those things are, that A—
(a) penetrates sexually, by any means and to any extent, either intending to do so or reckless as to whether there is penetration, the vagina, anus or mouth of B,
(b) intentionally or recklessly touches B sexually,
(c) engages in any other form of sexual activity in which A, intentionally or recklessly, has physical contact (whether bodily contact or contact by means of an implement and whether or not through clothing) with B,
(d) intentionally or recklessly ejaculates semen onto B,
(e) intentionally or recklessly emits urine or saliva onto B sexually."
You said that there wasn't penetration, however mentioned "rubbing" which would fall under subsection (b) as - according to your account - there is no reason to suggest that the doctor should have been anywhere near that particular region of your friends body, nor rubbing whatever part that that may be.
I'd definitely consider encouraging your friend to report the matter to the police. You cannot provide a statement on their behalf, unfortunately.
It would rest on the investigating officer to gather enough evidence to prove a crime has been committed.
Again, with the lack of information in your post its hard to gauge how successful such an investigation would be, however there are a number of avenues to go down whether it be witnesses to the incident - nurses or fellow medical professionals, forensic evidence available (dependent on timeframe and the manner of the touching), interviewing the doctor or maybe similar incidents have been reported before.
There is a chance nothing may happen, however even reporting it may raise the possibility of preventing similar happening to other people.