I think that was nice of you to check this guy out for your friend. There is an age difference here between a 12 year old girl, and a 15 year old guy after all.
But, if they are actually dating, you have two choices as to the email between you and him. You can either tell your friend you were checking him out, and what your impressions of him are, or, cut the communication with him entirely and risk your friend finding out.
I would recommend you tell your friend what you've done, and cut ties with him yourself.
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