There is a great organization called Resolve with chapters all over the place - they specialize in infertility and everyone there can relate. There are support groups but also on-line help and referrals and medical info and....much more.
I don't think all infertile women resent their friends with kids or feel they can't relate. One thing that I HATED when I went thru infertility was when people said, "Oh, be glad you don't have kids because today my little devil did....." (fill in the blank - clogged the toilet, dumped out my shampoo, burned down the house...) or "You're lucky you don't have a child because at least you can sleep late." That stuff really stings.
There's not much you can do for her but be sympathetic. Find out if she wants to talk about it, or if she prefers not to. If it helps her to have a ride there, you might offer. Anything else that lightens her load in other areas of her life. The meds she takes can make her moody or tired sometimes too.
Perhaps even if you go in with a few friends and buy her a spa treatment (a gift cert she can use for anything - like massage or facial, whatever), that can be nice. Even a nice "thinking of you" card can be nice.
She also needs to leave a normal life - she doesn't always need people talking to her about her infertility. Share other interests, make her laugh, etc. Not when she's devastated, but in between treatments. None of us want to feel like we're walking around with a big "I" for "Infertile" on our foreheads - we are more than a uterus, you know? Maybe a "Girls' Night Out" with dinner and a funny movie or chick flick? Figure out if she's allowed to drink alcohol - sometimes you can, sometimes you can't, sometimes it matters where you are in your cycle. Plan around that as needed.
There are some things she can do nutritionally to help balance her body out to help the treatments take and contain the endometriosis. I would be happy to help with that but I'd have to work directly with her. It won't conflict with anything she's doing medically.