It sounds like we're all on the same page in this thread... The best way to support your friend is simply to listen. Sometimes people just want to be heard so provide that support system for your friend. Don't offer suggestions or advice unless you are specifically asked and even then be careful of offering help. If your friend is an abusive relationship, provide the information (phone numbers, shelters, etc) that can help her leave her abusive situation. Otherwise, simply listen and be there to offer a hug or a word of encouragement.
Seeing a friend go through heartache is not easy and sometimes you can take that burden into your home. But like InLoveWithLove said, you need to be there for her and support her as much as she needs. You can also give her little tips that have been known to work for you in your relationship, although their relationship may not be similar to yours.
The best thing you can do is to be there to comfort your friend when she needs a should to lean or cry. I wouldn't suggest giving her advice unless you're a licensed professional because he or she can blame you if the advice you gave, does not work. This could end up putting arift between the two of you. So to be safe, try to avoid doing that.
The best way you can help a friend is by giving your complete attentive ear. Help him or her to dig deeper inside and figure out what is causing the pain in their marriage. Ask a lot of questions and get him or her to keep talking. If you have a partner yourself perhaps you and him or her can help them to have a conversation together and share what’s in their hearts with each other. The best thing you can do is just be a support, provide the help you can, and maybe suggest a therapist if it seems like it is something they are unable to solve on their own.