If it were me, I would direct the guy's girlfriend to this website if she is somebody who finds on-line support helpful. I found it incredibly helpful in assisting me to gain some understanding of poly when I met my SO and he told me that he had had several poly relationships and feels very positively toward that style of relationship.
After reading here lots, talking to friends who have been poly and going to some talks on the subject, I came to the conclusion that while I see the benefits to some and understand how wonderful it can be, it isn't right for me or for the relationship I'm in just now.
Things might change in time but for now my SO and I are monogamous with each other. With the understanding that if he at any time needs to be in a poly relationship again, we can shift to being friends so that both of us are able to seek whatever relationships we believe will help us be more happy.
I found this site invaluable in helping me resolve my thoughts and feelings on the matter. It's hard when faced with somebody talking about how wonderful poly is to be able to say no to it without feeling as though you are being controlling and close minded. This site is a valuable source of wisdom and experience about why poly may not be wonderful for all people at every time in their life. It helped me lots.
If the guy's girlfriend doesn't find internet support useful I'd suggest making it clear to her that she should talk to her friends and family about what's going on. Finding out that the guy she lives with and loves has been lying to her about a significant part of his life for 18 months is going to be a blow to her. She will need love and support and may well not want to seek those things from people who have been lying to her for so long.
For yourself, I'd also suggest caution. It is not at all uncommon for people who are willing and able to lie to those closest to them about major things in their lives to do it to everybody in their lives. Be aware that you don't have the full picture. You only have the word of a person who is willing to lie to get what they want.
You wouldn't be the first person to start a relationship while cheating only to find that the person who was willing to cheat on their existing partner with you is equally as willing to cheat on you with somebody else.
Poly is no safeguard against cheating. Have a read of these boards. It isn't unheard of for people who have a habit of lying to and cheating on partners to continue doing so even when they are free to have multiple relationships.
Good luck to all of you.