It is common practice to allow drinking coffee pretty much anywhere. It is also widely accepted to drink unflavored beers. Would that logic allow one to drink beer that is commercially brewed and/or flavored with coffee?
Second ShimonbM's comment. Practically, as soon as we hear "flavored beer" we get worried what might be in there. Without any further information I wouldn't buy a non-hechshered "coffee-flavored beer" at the store for that reason. However if you're certain the only ingredients are wheat, barley, coffee beans, hops, yeast, and water, and the equipment isn't used for anything else (are you brewing your own?), I can't see the issue.
(With regards to both coffee and beer, the question is raised [Kitzur Shulchan Aruch 38:12] why they're not prohibited under the ban on gourmet-food-cooked-by-non-Jews. The answer given to both is that the main ingredient is water, which doesn't need cooking! I thus can't see how coffee-with-beer would be any different.)
Coming at the answer from a different angle, I once asked a brewery about a coffee stout and what other ingredients might be in it. I received this response:
I don't think the Coffee Stout would fit the Kosher diet as there are food grade dairy sweeteners that we purchase. The coffee is not made by us but is on dedicated equipment.
This confirms the concern other answers have expressed that once a beer is flavored, it opens the possibilities for many other unknown and possibly non-kosher ingredients.
I have asked both the OU and the CRC about this. It would make sense that since unflavored beer doesn't require a hechsher and certain ingredients themselves don't require one that beer with them shouldn't require a hechsher. The example I asked about was coffee. The answer from both was that beer companies need to make it known when they flavor their beer and once it is flavored then it is possible there was also flavoring that was non kosher added as well. Here are the email conversations I had with them...
The standard for beer has always been than unflavored beer requires no certification but flavored beer does. Understandably from the understanding some flavors are created by non kosher sources. However, I have not come across a good answer as to why all flavored beer requires certification as many of those ingredients added on their own wouldn't need certification such as coffee, coriander seeds, or orange peels. Thank you for your help, it is greatly appreciated.
The OU's response:
Thank you for contacting the OU.
The flavors come as industrial flavors. All flavors require certification. Please do not hesitate to contact us again should you have any further questions.
To the CRC
So the standard for beer i have heard repeated many times is that unflavored beer does not require a hechsher but flavored beer does. This makes sense in many cases but I have to ask why for coffee beer? Coffee beans don't require a hechsher and so why would a beer that has coffee beans put in it require one?
The CRC's response...
Thank you for contacting the cRc with your beer question.
All flavored beer requires hashgacha due to kashrus concerns. Once a company adds even a simple flavoring, we don’t know what else they are adding. Hatzlacha,
Rabbi Akiva Niehaus
My follow up question to the CRC...
By that logic theres no way to tell if any beer without a hechsher added something extra in. My question is about when they have added ingridients which don't require a hechsher such as coffee?
The CRC's response...
Beer which has added flavorings must declare so on the label. Once they declare that they have flavorings, there are two issues – perhaps the stated flavoring isn’t kosher and perhaps they are adding additional flavorings which may also not be kosher. Rabbi Akiva Niehaus