I don't think you should've felt bad for unintentionally offending her. Really, what else could you have done? If your mother had assumed her religion and told her which foods she could and couldn't eat, and it turned out she wasn't of that relgion, she might get offended.
If I know someone is allergic to shellfish, or vegetarian, I will point out what they shouldn't have. But I wouldn't think "hmm, that lady kinda looks like she is of the *ABC* religion, I should tell her what to avoid", because that makes assumptions about their appearance, and how strictly they follow their religion etc.
Your mother did just fine! In most cases, the truth is always best!