NRE can be brutal, especially if someone is new to polyamory, but even if they are experienced it can whirl you away in the hormones and chemicals.
If she's serious about being ethical and thoughtful to the romantic partners in her life, I'd seriously suggest she read everything possible on NRE (new relationship energy), and also about introducing poly into an already established relationship. It is her responsibility to learn what NRE can do (cause her to make really bad judgments, to be self-centered and to neglect the rest of her life), so that she can see when that's happening and slow herself down--and so she can be sure to attend to her marraige at the same time. This can be difficult, but in the end if she doesn't she may end up losing her marraige and then wake up from NRE and wonder what happened.
Also, when introducing poly into an already established relationship, you have to go a bit slower so that the person who is basically having their whole life changed can adjust, learn, deal with feelings and learn to be comfortable.
If I had one piece of advice for people who were introducing poly into their relationships it would be for the person who has the new relationship-- Take Care of Your Partner. Be loving, be understanding, be caring-- let them know you will go at their speed, let them know you understand their concerns, let them talk to you about what those concerns are. By doing that you are showing them that you love them and want them to be in your life and are willing to do the work to help them get there. If you don't do this, and do things that break boundaries, or push to get what you want, lie to them, or just do what you want no matter what they feel because you don't "get" their emotions-- you are showing them that they are NOT your concern, that you are more concerned with fucking some new person than in maintaining your relationship, that they cannot trust you, and that they are your last priority over yourself and the new person. This will make poly really hard to do, if you even can, and may very well end up in you losing your relationship.
I'm not saying cater to every whim... your partner needs to do some work as well, and be communicative and take responsibility for their own feelings. But if you are there to support it, instead of rush it and be impatient because you want to go off with someone else, you have a better chance of success, and an easier time of transition.