UPDATE: Thanks everyone for your helpful thoughts so far! I guess to jump to it, if you have any advice for if my friend should stay or leave that would be helpful. I know she really likes this guy, but we're all new to ENM (I've been actively dating ENM for a few years, my friend a little shorter time than me, and Fred has actively poly the shortest amount of time), so I don't know what is good advice for her in this situation. If you think she should stay and this can be worked out through convo, let me know! If you think she should flee for this hills, let me know!

I saw an earlier post with a reversed scenario that made me realize a question I’ve been mulling over for the last few weeks might be a good one to be answered by a community with cumulatively more experience than me. 

Title kind of says it all - a friend's partner is dating someone else who is very much monogamous and I'm curious if this is common. My friend and her partner are both poly and have been dating for a few months. Both have other partners and are transparent with each other. However, my friend's partner (Fred, M30) has recently been dating a woman who is monogamous. This already had me arching my eyebrows, because I assumed that they had incompatible relationship styles, comforts, and goals. Last I'd heard from my friend, they'd decided to call things off, but are still "dating", as in texting a lot, flirting, hanging out, but aren't sleeping together. I'm having a conversation with my friend about how she's feeling about this and how she's processing everything, but personally, I don't know how I'd feel. Fred talks to and spends more IRL time with this partner than with my friend, and the monogamous partner has apparently been consistent that she would like a relationship with Fred, but doesn't want to do ENM.

For those who have been poly for a while and may have been in situations like this, do you have any advice for navigating a situation like this? Is it common to date or be with a monogamous person even though you're poly?