Treat them normal and be patient when they are speaking. It may be uncomfortable for you at first because it can be socially awkward if you are not used to it. But, be patient and give them your full attention when they are talking. Don’t finish their sentences because they know what they want to say. It just takes a little more time to get out. Ignore the stutter. Don’t mention it unless they bring it up. And, most importantly, thank you for caring and being self conscious enough to ask for advice.