[arlingtonguy]

A condom is not an absolute. It is no longer called safe sex, rather safer sex because no sex is 100% safe.

At 16 I assume your friend has not had much sexual activity so there will most likely be some tearing and trace amounts of blood involved. This is normal. Lube will definitely decrease the risk of tearing and wearing a condom with prevent any blood transfer..
Also you should recommend that your friend uses a warm water enema to clean out any feces that might be in the rectum. You can buy these at a drug or grocery store. The best to use is called fleet. It is a clear plastic bottle with a nozzle on the end. Empty the premixed solution and refill the bottle with lukewarm water to help relax the bowel. The solution in the bottle contains detergents that can irritate the rectal lining so empty it and use lukewarm water. This will make it more comfortable for him and less messy for you.

I would also suggest having your friend be on top straddling you as opposed to the missionary position. That way he is in control of the insertion and can determine how deep he can take you comfortably. As tempting as it may be do not use drugs or alcohol of any kind. Pain is the body's response telling you something is wrong. If you dull those pain sensations you could really injure your friend.

If you go slow and just have fun with the experience it should be a pleasureable and memorable one.