Drinking a tall boy right after your Botox might not be the best idea in the world, but in reality it shouldn't affect your long term result. At most, it MIGHT make you bruise a bit, since alcohol tends to make you more prone to bleed. If you have a really good time and end up passing out early, gravity can alter the diffusion of the Botox, which is why your doctor likely gave you post-injection instructions that told you not to lay down immediately. Either way, it shouldn't be a big issue, everything in moderation.
It is not recommended that you consume alcohol for the first 24 hours following Botox injections. Alcohol can thin the blood and make you more prone to late bruising. It is unlikely that Beer would reduce the effect of the Botox and unlikely to cause health risk. To be on the safe side, however, you may want to wait until tomorrow.