There is no single answer to your question. Instead, the answer depends on the specific benefit, and also may depend on whether your mother-in-law is a U.S. citizen. Here is a run-down of the possibilities.
Medicare. Medicare does not provide any coverage (except for brief, emergency care in very limited circumstances in Canada or Mexico) outside of the U.S. and its territories.
Food stamps and Medicaid. If your mother-in-law is receiving food stamps, she may also be enrolled in Medicaid, but there is no food stamp assistance or Medicaid coverage outside the United States.
State disability benefits. Your mother-in-law may be receiving disability benefit payments from the state where she lives. But these benefits are only payable to residents of the state, so she will not continue to receive them once she leaves the U.S.
Supplemental Security Income (SSI). If your mother-in-law is disabled and has very low income and few assets, she may be receiving monthly benefit payments from the federal government's Supplemental Security Income (SSI) program. However, these payments cannot continue if she moves out of the U.S.
Social Security benefits. If your mother-in-law is receiving benefits from the Social Security Disability program, she might be able to continue receiving them if she becomes a resident of another country. If she is a U.S. citizen, she has a right to continue receiving Social Security benefits -- including disability -- in almost any other country. Even if she is not a citizen of the U.S., in most countries she can still receive Social Security benefits. The rules for receiving Social Security benefits while living abroad are explained in the Social Security Administration web page Your Payments While You Are Outside The United States. If she continues to receive Social Security Disability benefits, she will have to regularly provide proof to Social Security, usually at a U.S. consulate in the country where she lives, of her income and her physical condition. Once she reaches full retirement age, she could switch from disability benefits to the same amount in Social Security Retirement benefits (if she qualifies for them), which would mean she would automatically receive her benefits and would no longer have to regularly check in with Social Security.