It is great that you want to continue to support her It is most important that you let her know that you are there for her and that she can talk to you when she needs to. It's also great that you are seeking information to understand.

I understand what cutting is, but I do not understand why people do it?
People start self-injury for a range of different reasons and even the reactions they have from harming could be different. Some do it from shame, some to release pain, some to feel some form of emotions. Is this something she is willing to discuss with you? She may not be ready, but in time she may start to be able to tell you the reasons and what 'triggers' her (or causes the urge for her to self-harm).

What is letting?
Letting started out as a cultural ritual - it is used is Polynesian communities as a rite of passage to clear toxins etc. It usually involves using needles or other utensils to remove blood from the body. The traditional methods were much more aggressive and I'm happy to provide information about how it works if you want it, but I will do it privately as it can be very triggering.

It is quite an uncommon form of self-injury but it does happen. It is often a way for people to feel 'real' if they are dissociating. It reminds them that they are still human and still alive. It can also be used if people feel 'dirty' or 'unclean' as a way of trying to purify themselves.

I'm at a loss as I really cannot handle blood, scars, gore, and things like that.
A lot of people who self-harm often also struggle with this, or did before they started to harm. The self-injury is often more about the release of pain than the blood etc. I know of some people who self-harm to manage high levels of pain but still can't stand the sight of blood.

My question is is there anything that can be done to help her?
As Sam said, the best thing you can do is to continue to be there for her. Let her know that you are there if she wants to talk or needs you but try not to push her too far until she is ready to talk. I would also agree that it would be good to encourage her to seek help. Therapy to work through both this and her other mental health concerns can be extremely helpful.

Also, make sure that you continue to look after yourself. It is great that you are willing to support her but if you don't take care of yourself you can also burn out. Just make sure that you still have fun and do things that you enjoy.

Or this something that is continuously in her life and will be dealing with for the rest of her life?
There are different opinions about overcoming self-injury. Personally, I see self-injury like almost any other addiction. It can be overcome but there are still times, when stress takes over, that the urges come back. In time you find ways to manage them though, with the right help and support they aren't as strong and you find ways to cope without the self-injury.