How long should you get in prison for street robbery at crown court in the uk?

Hi a friend of mine was beaten up and robbed by 3 youths they knocked his teeth out stole his ipod, phone,watch. 2 of the boys got let of but the one who did rob my friend went to prison for 8 months which i find disgraceful and im just wondering if thats the sentence for vile people who feel the need to rob innocent people.thanks