Well, I don't think you need to do something specific. Just act like it's normal. If s/he has bad block, just let him/her get through it. I know my ex-gf never paid much attention to it, and I was happy with that. I'd block, she wouldn't act like it's something wrong/different and pretty soon I got comfortable around her and didn't mind if I stuttered. Now, if it's particularly long block, then I don't know. It is kinda weird just standing there looking them in eyes while they are struggling. No idea how to approach that. In the end, ever person is different. Just talk about it openly. If I've learned anything in my relationship is that being open with other person and discussing things on your mind can only strengthen your relationship.