You are correct. ARVs should never be injected into the gluteal muscle (buttocks). Fatty tissue in this region slows absorption, leading to a weaker immune response. As rabies is 100% killer, so you need urgent medical attention.
Next Steps
If a rabies vaccine has been given in the buttocks, you must seek immediate medical care to have the dose repeated in the correct anatomical location (the deltoid muscle of the arm or the thigh).
Health Tips
Do a Google search: whether ARVs can be taken in buttocks? your doubts will vanish and you will rush to complete the course from Day 0.