OMG!!!! Your situation is almost parallel to mine. I being the wife was the one that aprroached hubby about polyamory after falling in love with someone else. We've got NRE all over the place and are really trying to take things slow. I can also really relate to your concerns about your wife's friend being so young. My husband's new love is only 18 and we haven't met her in person yet either. I am very concerned about her maturity level and age with this kind of relationship structure. But so far things are going well for us. We are just taking things a little at a time.
Good Luck to you