My friend deleted my comment on her post. A mutual friend of ours is mad at me and hung out with my afforementioned friend today. Could this mean they both are mad?

Open and honest communication is usually the best way to deal with most situations. By speculating about whether your friends are both angry with you, you are assuming. Assuming is a one sided thing and does not include open communication at all. After all, you do not know what they would even be angry about, so even if you knew, it would not really be helpful to you. The only way for you to know for sure whether them both hanging out together has anything to do with you is to communicate with your friend. Assuming based on behavior can be destructive to your mental health and the people around you.