Hello so my sister's Crassula perforata leaves are falling of and the bottom half has no leaves at all! what should i do with the bottom half? should i just leave it be and its also is leaning to the side does it need more sun? (direct sunlight or not?)

This might just be a normal part of aging for this plant. Maybe it's time to propagate some new ones? Here's an article that might help.
https://needlesandleaves.squar...
I know these plants need a lot of light, but indirect light is typically best; however, someone closer to your growing zone may be able to provide better information.

Maximum indoor sunlight will help new stem growth to be thicker and stronger and better able to grow upright. However, even in the best conditions, these plants will eventually succumb to gravity and start to lean and also lose lower leaves. The solution is to prune the leaning and elongated stems back to a height of 1-2 inches. New growth will then emerge just at the point where you make the pruning cuts and grow upward from there. You can propagate the cuttings in porous potting mix or water.

The plant needs more sun but a gradual dying off of the lower leaves is a normal part of the growth cycle. Indoor sun is not direct because regular window glass cuts most of the UV. I agree, as much sun as possible indoors is ideal. Here are a couple of pictures of my plant currently growing on a rooftop patio. As you can see it does tend to grow sideways given enough time, even in lots of sun, and I don't mind the unkempt look myself. The flowers are terminal and leave a dead stem just below when they die off, so you want to prune that part eventually (obviously I am lazy about this).

The plant should be very easy to start from cuttings. Start in soil for the best results.