I generally shy away from giving advice on an open forum, however;
If the OP's friend witnessed a serious criminal offence and has 'material' information that the police are not aware of, and he/she refuses to tell them, he/she can be charged with concealing a serious offence.
I would suggest that your friend contact a solicitor before saying anything to police. I am a criminal defence specialist and more than happy to provide advice to your friend if required. PM me for my number if you wish.
The 'right to silence' as referred to by others above does not necessarily apply to witnesses.
Cheers
Chris