Sorry Vandalin..here is something I just wrote on another thread. Not that it will take your pain away but your friend sounds very mono to me.
As a mono person I can pretty much guarantee we develop differently. I doubt that the pairings your ex had were any more loving but they probably came with a sense of security whether artificial in her cases or not. It's just nice for a mono person to know or at least think they are enough for the person they love.
All of us have to admit that polyamory fills a void, completes ones ability to love or expands it. It's not all about "I have so much love to give" it's more about "I need more Love".
To a mono person, this hurts on a primal level. It can either be overcome by the degree of love in the relationship or it will be too much and they must go their separate ways to remain healthy.
I have always said I would almost never recommend a monogamous person get involved with a poly person and I stand by that. I have a wonderful relationship with Redpepper and yet I know I would not seek out or accept the love of another poly person if we were not together.
If you don't have the capacity to intimately love more than one person it is very hard to accept it in your partner.
Nothing can take away some pain such as a break up except for time, time and more time.
I hope you feel better soon Vandalin..take care