Just to elaborate on SleepyJ's answer:
Where the subject of a photograph is aged 16 or 17, the law is not as restrictive as when the subject is under 16. (So photography of simple nudity might not automatically count as an offence). However, when the person pictured is under 16 years of age (and the pictures are not, say, just innocent images, taken by a parent, of a toddler in the bath) then possession and distribution of images of nudity almost certainly constitutes a breach of the Sexual Offences Act 2003.
Illegal images are grouped, by case law (R v Oliver), into five categories. Simple nudity is at 'level 1' but, if the images are posed, this could take them into 'level 2'. If there is any sexual content to the photos (e.g. concentration on genitalia), this could cause the images to fall into level 3'. (Levels 4 and 5 are used for penetrative sexual acts and masochism).
If the police were to get involved, the first problem your friend might experience would simply be the hassle of having his home searched and his computer taken away for many months. (The last time I saw a press reference to current processing times, it was suggested that the police take about 9 months to examine a suspect's computer). If charges were to follow, a conviction for level 1 offences would probably be unlikely to result in a custodial sentence. (It couldn't be completely ruled out. This is because of the 'distribution' of the pictures). However, if the photographs were at level 2 or 3, the possibility of a custodial sentence increases. Either way, your friend would be placed on the sex offenders register and have to notify any changes of address, or holiday plans, to the police for several years. He would also be barred from some types of employment for life.
Chris