Will Medicaid stop paying for my grandmother's assisted living facility if we take her out for the weekend or out for dinner?

Absolutely not. In determining someone's income to decide whether they qualify for Medicaid, substantial regular non-cash contributions from family, for example in the form of rent-free housing or daily meals and all clothing, is sometimes considered. But even regularly taking your grandmother out of the nursing home for a family visit or getaway, or for a meal, does not amount to "income" and would not disqualify her for Medicaid coverage of her nursing home care.