People should always pick up their own friends, lovers and spouses-why? Because if you test a person yourself chances of your fitting in with that person are far higher.
We all do it, mingle with the friends of our friend, or siblings and mostly after some time we discover that it was not that good an idea. Sometimes we end up regretting it, especially if our relationship with the person who introduced us to that group is ruined, we often discover that his/her friends quite naturally are more loyal towards him/her; or worse, we discover through his/her friends that s/he was not what we thought s/he was! That is a different issue, but in the end, it is always better to spend your time with those who you have decided to make a part of your life, share your time with.
Even your best friend’s friends may not be of your league, or they may get jealous of the special bond you share with your friend and try to ruin it. That too happens very often!