First off, welcome RockBird. Thank you so much for coming here and sharing your story with us. I'm sure that are large chunks of it that resonate with the folks here.
These types of issues are really tough to deal with, and made even tougher when the people involved are folks you care so much about.
I'm going to ask you a question for clarification - maybe you already know the answer, maybe you don't and need to think about it - the idea is to get a better understanding of what is going on.
I strongly believe that we are "wired" the way we are, whether mono, poly or some variant - trying to "become" the other to try to make someone happy isn't going to work in the long run and is only going to cause unhappiness. So please don't feel badly that you can't "be poly" for this person. The fact that you know this in your heart is a major step - so many people are just confused and bounce around.
So, that having been said, when you say you are "not poly", does this mean that you wouldn't want to have a relationship with other folks, or does it mean that in addition to that you don't want your partner to have relationships with other people? There's a big difference between these, and the answer will obviously have big consequences.
If it's the first one, then your and her needs are fundamentally incompatible - that's not a slam on anyone - it's just a neutral fact. trying to have a relationship under those circumstances would probably be very unfortunate.
If it's the second one, then you could imagine yourself as part of a "V" with her and your friend.
Very different possible outcomes for that subtle question.
If you end up realising that this isn't for you, then you have every right to set some boundaries as to acceptable behaviour - it's your house and you have every right to be comfortable in your own home. Try to think of your boundaries - gauge your level of comfort on things like:
- Her coming over and visiting to watch TV with him
- Her cuddling on the couch with him while you are there
- Her staying the night with him
- Her and him having sex with him in his bedroom
- Her and him having sex in the living room
and so on. Some of these might be hard "no"s and some might be "I'm not sure". Once you get that worked out for yourself, you will need to talk with them about it at the right time.
Wanting everybody to be happy is a noble goal, as long as it doesn't come at the expense of your happiness, which wanders into the realm of co-dependence.
I wish you luck!
"Listen, or your tongue will make you deaf." - Native American Proverb