The Quran explicitly states that the food of the People of the Book is lawful for Muslims, a ruling classical scholars primarily understood as referring to meat slaughtered by a practicing Christian or Jew in the name of God, in addition to their general food. Modern industrial meat production complicates this somewhat, since large-scale commercial slaughter, whether labelled kosher or not, often no longer resembles the traditional individual slaughter the early scholars had in mind, and mechanized or stunning-heavy processes may not meet the conditions either tradition originally required. Because of this, many contemporary scholars advise caution specifically with meat from unverified sources, while remaining relaxed about vegetarian, dairy, and other non-meat dishes prepared by Christian or Jewish friends, family, or coworkers. When genuinely unsure about a meat dish's origin, it is fine to simply ask or choose a vegetarian option instead.
Q&A · Health & Halal Food
Is it permissible to eat food from the People of the Book (Christians and Jews)?
References
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.