Your friend should talk to her husband asap.
You guys may find useful thoughts about basic principles for healthy poly at www.morethantwo.com
Go for it!
As Nyc said, we'd really need specific questions to give more feedback.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.