Perverting the course of justice (or attempting to do so) is always taken very seriously by the courts but the likelihood of a prison sentence is, to some extent, determined by the severity of the situation to which the offence relates.
For example, some people try to evade speeding charges by lying about who was driving the vehicle. This can lead to a short period of imprisonment, as here:
http://news.bbc.co.uk/1/hi/england/beds/bucks/ herts/6480005.stm
However, other people in roughly similar situations might avoid prison, as in the well-publicised case of the 'fictitious Bulgarian':
http://www.timesonline.co.uk/tol/news/article5 78756.ece
So, when the charge of perverting the course of justice relates to a relatively minor original offence (such as speeding), it's possible that a non-custodial sentence might be passed. However, in the Soham murders case, Maxine Carr was prosecuted for the same offence because she provided Ian Huntley with a false alibi. It's not hard to understand why a lengthy custodial sentence was passed.
The person you refer to probably has a reasonable chance of avoiding prison if the original offence was fairly minor but a virtually non-existence chance if the original offence was very serious.
Chris