Support. You may help explain certain things such as acne, menstruation, erection, hair growth, voice changes, etc., however, you don't want to do that in a way that may get them offended. You may come to their doctor along with them if they ask you to. Also, you want to be a good rôle model for them if they experience behavioral challenges at school or/and home to help them steer away from trouble.
Be supportive. Puberty is the culmination of a lot of changes that start sometimes about 2 years or more years earlier. Your friend who i assume to be a pre teen or teen should have received some education on what changes to expect both physically and emotionally either through his/her school nurse, parents or pediatrician. If not, alert your friend's parents to do so for him/her. In the mean time, be supportive.