Real; some tips:
Don’t try to prove their obsession (s) wrong
Don’t say ‘calm down’
Don’t say ‘what, are you worried about crossing the street too?”
If you’re able to be a listening ear, go for it. If you’re not, then politely say that you wish you knew what to say and how to help, but that you don’t know, but that you respect them and don’t think any less of them because of their condition