Colloquialisms, idioms, and figures of speech are all perfectly legitimate uses of language in an informal setting. I think we can all agree brown cafe qualifies as informal. It's unnecessary to correct someone's use of language when what they wrote got their point across.
I wouldn't worry so much about presenting yourself as well educated. If you truly are, then it will come through without needing to prove it by trying to correct others' use of language unnecessarily.
Here's an article that should help:
Caring About Whether You Couldn't Care Less - Everything After Z by Dictionary.com