Lipreading is a vital tool that deaf people use to them to understand what others are saying but only 30% of information is lipreadable.
When someone is lipreading, they are putting together the bits that they can hear (some people might hear some sounds and not others), what they can see in terms of lip patterns and shapes while also picking up clues from the environment, including your facial expression, gestures, and what is going on around about. This is a lot of hard work and is prone to error, but there are some simple things that we can all do to help someone to lipread when we are chatting with them.
All of these will make a big difference, and will help the person feel like you are on their side and want to communicate with them.
Most of all, be patient and kind!