How can I help my friend deal with going through puberty?

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.