Chesty:
Suggest your colleague gets a printed copy of
http://www.caa.co.uk/docs/33/FOD200328.pdf to keep.
Section 5.1.1 shows that legislation allows police officers to require a breathtest. Whether second hand reports of an "alcoholic smell" in a place where the police officer hasn't been counts as "reasonable suspicion" is dubious, in my mind. There is a power to board an aircraft if there is reasonable suspicion, however.
However, Section 5.1.4 is the one your colleague needed to be aware of. He was well within his rights to request it to be done in a non-public place, if my reading of the FODCOM is correct, but has no actual legal right to that. However the "ways and means" act of getting around this is may be to offer to provide the specimen but only in a private place - that way, there is no refusal.