My friend for past year has been obsessed with her body weight, she has lost loads of weight and she looks unhealthy.
She never eats around her friends and ive only ever seen her eat a peice of fruit or some nuts.

She once admitted that she feels guilty when she eats and that she starves herself, she also admitted to vomitting after eating.
But now she is totally in denial for ever saying that and just doesnt see what she is doing to herself.

Im so worried about her, does this sound serious to you?

What can I do? I realise that this is a sensitive topic to bring up with her and I dont know how to go about it.

It does sound serious to me, is there any adult you can talk with about this? Try to talk with her first and see if you convince her that she needs to talk with somebody. If you talk to somebody without her permission she might be angry with you, but it's important that she gets help before it get worse.
I had anorexia and I would to great lengths to avoid eating and to keep people from "interfering" with my self-starvation.

I she seeing someone about the ADD? I have an eating disorder, I've had it for 20 years, and nothing helped until I was, just recently, diagnosed ADHD, and medication has helped a lot.

If she is not seeing anyone about the ADD suggest it

Other than that the only advise I can think of at the moment (2:30am) is don't push it, or she will avoid you and push you away. Just let her know you deeply care for her, and that you are there if she ever needs you for anything and leave it at that for now.