Alhough a mother and a friend may both have love for the individual in consideration, these roles typically have significantly different approaches in how they think of and interact with their child or friend.
Mothers try to have their child's best interests at heart, especially when they consider long-term goals and security. While friendships also involve care and well-wishes, friends may not have as much foresight as parents when it comes to making big decisions. For example, if a person was really passionate about music, a mother would be more likely to suggest that this person get a degree in music, while a friend might suggest the individual take his or her band on the road. Both are potentially good decisions, but the mother's suggestion, in this case, would offer far more long-term job security.
The differing personalities of mothers and friends can sometimes create conflict in a person's life. Young people are still mentally and emotionally developing and need time and space to figure themselves out and experiment with identities. With that in mind, friends can have positive or negative influences on each other. Social deviation encouraged by friends can be a good learning experience, but also often brings heavy consequences. Mothers are less likely to encourage their children to participate in social deviation, often because they've learned lessons from their own experiments!
The relationship roles of mother and friend both carry the unfortunate fact that we often think of people as existing only in their relationship to us. It can be hard for a mother to imagine her child as anyone but the specific person she knows, and vice versa. I can certainly say I sometimes have a hard time imagining my mother as anyone but my mom. Friends, too, may be limited in their understanding of a person. I would argue that where mothers and friends differ in their understanding of a person is that a mother is more likely to know her child's fine-tuned emotional nature, while a friend is more likely to know a person's outward, socially-presented, personality.
In general, I think mothers take far more responsibility when it comes to their child's actions and how those actions might reflect upon them. A friend may feel a sense of distance from a person whose actions do not reflect upon them personally. When things are seriously tough, a friend might even have the option to walk away from the friendship, while a mother-child relationship is for life.