A caution isn't a criminal conviction, so (irrespective of the time interval), there would never be any need to declare it in response to a question, on an application form, which asked about 'criminal convictions' or a 'criminal record'.
Because a caution isn't a conviction, there is no provision, under the rehabilitation of Offenders Act, for it to become 'spent' but it seems likely that the Criminal Records Bureau would apply the same criterion to cautions which applies to non-custodial sentences: i.e. a caution is likely to be treated as if it were 'spent' after a period of five years (or for someone like your friend, who was under 18 years old at the time of cautioning, 2.5 years).
This question came up earlier this week and I provided a much more detailed answer, with relevant links, here:
http://www.theanswerbank.co.uk/Law/Criminal/Qu estion324852.html
Chris