Depends on the circumstances.
Did they come to me as a client/patient or did they come to me as a friend?
If they came to me as a friend, then I couldn't provide advice due to ethical and legal reasons. If I provide advice I could be held liable for advising them to make their decision, of which, complications may arise, and in that instance insurance won't cover me and I could end up in deep shit. Best I could do is carefully discuss the risks but not advise them on which decision to make. If the pressed me for an answer I would suggest they consult their family/regular GP, or better yet, their pediatrician.
If they came to me as a client/patient, I would discuss the risks and possible outcomes of both choices and whether it is a necessity. My honest advice would be to avoid circumcision except in the instance where it is required for medical purposes (extreme case of phimosis would be one example). Basically, if it isn't broken, leave it alone! And as above, I would suggest they consult their family/regular GP and/or their pediatrician.
Another question: Are they doing it for religious/spiritual reasons?
I can't discriminate against religious/spiritual reasons for the procedure, but that doesn't mean I need to advise them to do it either. As with above, I would discuss the risks/outcomes and still advise them to leave it unless circumcision was medically necessary.
All in all, as a doctor, and in this particular case, I can only advise, not instruct, so the best I can do is be helpful and provide as much information possible without discriminating.