My husband would also NEVER take lessons from a professional instructor. Just not something he would do because he is stubborn. We are not at the point of me teaching him how to ride because we don't currently own our own horse, however when the time comes, I plan do pretty much what everyone has said here.
You just have to remember to be patient. Instead of telling them everything they are doing wrong, try to put a positive spin on everything. "Your leg position looks great! Remember to keep those heels down". Something to that effect.
Also, I am not sure if you are working with them both together, but you may want to keep them separated until they both get a general grasp of everything. If your bf is anything like my husband, he will get embarassed quickly if anyone else is watching him ride. Working with him alone will make sure that he doesnt get overly defensive about anything and you will be able to keep your eye on him and only him.
Good luck :)