BornToFolk, I've started to post several times and don't really know how helpful I can be, but I want to say how sorry I am to hear what you're going through and how glad I am that you're posting here because the support you'll get is invaluable.
The short answer to your OP is yes, it is possible, but FridasBrow makes a good point. In the circumstances, 'friendly' is what you should be aiming for rather than 'friends', because it's really important that you protect yourself until you start to get over this horrendous shock and can begin to trust your H a little again (as a co-parent rather than a partner I mean).
Now for the long answer (I suspect very long!)... I split up from my ExH about a year and a half ago, and I would say we have as friendly a relationship as can be expected, given that our marriage ended because of his affair, and that he is now living with the OW. He sees the DC at my house - they do stay at his place regularly at weekends, but his working hours rule out weeknight stays, so this way he can spend an hour or so with them every two or three days to keep up contact. We attend school events together, occasionally have family meals (eg for birthdays or if grandparents visit), and will sometimes meet for coffee or lunch on weekend days when I have the kids if it means they won't otherwise see him for a few days. At the beginning we did do one or two days out together, but not now - I found it too painful to pretend we were a normal family and felt like I was clinging on to something that just didn't exist any more. In general, I feel like we more or less have a good balance for the kids' sake. They see us getting on well, they see plenty of their dad, and we have a very relaxed approach to access which means the kids don't miss out on any activities/parties/family visits and I get time off when I need it.
However, there are two or three important things to consider. Firstly, it is very difficult to allow the person who hurt you so deeply to still be a big part of your life, and it takes time and a lot of healing to reach the right balance there. Some days I felt like 'wow, we're getting on great, surely there's some hope that we can be a proper family again one day' 
; other times, I could barely stand the sight of ExH and felt an idiot for letting him have so much more access to all of us than I thought he deserved. Now, it's more settled and the whole situation is bearable, but there are still ups and downs.
Secondly, although I still believe the decisions I made about his involvement in our lives were in the children's best interests, it has inevitably caused some confusion. The first few months were difficult for the DC just coping with the fact that daddy had left, and then they met the OW, which was a new aspect for them to think about and no doubt a bit of a novelty. But now, especially with my 7-year-old DD, there are a lot of questions. She doesn't really understand why I can't come with them when they visit daddy's house, or go on holiday with them, or why daddy and OW couldn't come and stay with us when they had to move house 
. Luckily she talks, she asks, we can discuss it (and indeed ExH and I discuss it too so that we can try to give her a consistent story). If she didn't, I'd be worried about how it was affecting her. Actually, I'm worried anyway, but at least I know what's going through her head and can try to help.
Finally, and probably most importantly, it's only possible to be friendly if it's an entirely mutual thing, with respect, boundaries and a whole lot of tact on both sides. If not, it's too easy for tension and bad feeling to creep in, and that's exactly what you're trying to avoid.
I hope I'm not making it sound like my situation is all rosy, because it's not, it's bloody hard a lot of the time; I often have to bite my tongue, I continually question my motives and wonder if I'm being too much of a martyr. But at the same time, I hope it gives you some kind of picture of what things could be like - if you feel that this is really what you want and not just a reaction to the hideous situation your H has put you in.
Whatever the case, good luck with it all, I promise you it does get better and it sounds like you're a strong person and a great mum - and if you're anything like me, your DS will give you purpose and focus that will drag you back up to the light like nothing else can!