I posted the same thing here:
only five minutes before you.
Here is the recent BATFE letter to FFLs.
Here's the last paragraph (edited):
Therefore, any person who uses...marijuana, regardless of whether his or her State has passed legislation authorizing marijuana for medicinal purposes, is an unlawful user...and is prohibited by Federal law from possessing firearms or ammunition...As such, you may not transfer firearms or ammunition to the person, even if the person answered "no" to question 11.e. on ATF Form 4473.
In fact, you do not technically even have to be a marijuana user. Just the fact you possess a medical marijuana card disqualifies you from possessing firearms and ammunition.