If you want to support someone going booze-free for Dry January, 'Huffpost' recommends avoiding these impolite questions:
"Can't you have just one drink?"
"Do you have a problem?"
"How will you de-stress without alcohol?"
"Do you miss it?"
"Are you pregnant?" (this one is classic)
If you really want to support someone during Dry January simply ask "How can I support your dry month?". You can even try to provide non-alcoholic drinks in social settings.