Constructive dismissal can only be claimed for if he was employed at the location for a period of 12 months or more. (Civil Law) If he was, then he should contact the Citizens advice bureau about how to proceed if he wishes to go down this line.
The matter regarding the "racist" comment comes under "criminal law" and anyone who hears anything they deem to be racist can make a complaint, even if it was by them overhearing a conversation. However with this particular incident, if it is as you say it is, you would probably find that the police would say it is not in the public interest to prosecute as there does not appear to be any malicious intent. Whether he is from Nigeria or Outer Mongolia it makes no difference. Any person of any race, or religion, can be classed as being a racist by their comments. Just because he may be "black or asian" and is making a comment about someone who is "black or asian" it can still be deemed as a racist comment. He just needs to be aware of this before making comments that could be overheard by a third party and misconstrued.