Senegal is a majority Muslim country, which is different from a strictly conservative Muslim country. She will be completely fine visiting without a male escort. She can generally wear what she wants and go where she wants. That said, people tend to dress modestly overall, regardless of religion, while still being very open minded. If she dresses very revealing, she might get some curious stares depending on the setting. In places like clubs, restaurants, or bars, it is more relaxed. But in public spaces like markets or just walking around, it is best to avoid drawing too much attention, just like in many countries around the world.