My first view - the relationships of the people I'm fucking, are none of my fucking concern. Partly this belief comes from living in a country where gun ownership is uncommon - and I'd probably have some more qualms if there was a high likelihood of a partner missing the fact that their loved one was cheating on them and aiming the firearm at me. But I don't give a fuck who someone I'm fucking is friends with/dating/married/etc. The ethical obligations of their relationships are theirs, not mine. I have fucked people who were cheating on their relationships in the past - and while in a slightly more mature mindset, I'm probably less likely to be attracted to someone who does that shit because they're probably not a good person, and because I can do without the drama or trying to fuck around with discretion, I still don't give a fuck about their relationships. If I met a good person who I was attracted too who I could fuck without needless drama or hassle - who was married or in a relationship - I'd still fuck them. That said, I do care about my relationships. I do make an effort to keep my ethical obligations to my friends. I generally wouldn't fuck a friend's partner without the consent of that friend. etc. But an ex? Well - it's unlikely that that would be the same kind of thing for me. I mean I probably wouldn't go there the week after the breakup. But a year later? It'd need to be a really, really fucking good friend or a really mediocre potential fuck before I'd pass it up after they'd been broken up for a year. Move the fuck on asshole. I also draw a pretty big line between friendship and friendly acquaintances. I have a shitload of friendly acquaintances - I have a much smaller list of friends. With friendly acquaintances - if the person they're hung up on wants to fuck me, there's a very short consideration of how much I want to fuck that person vs how much I care about maintaining the friendly part of the acquaintance. It's usually going to come out in favor of getting laid. With a friend - if they've genuinely got an issue with it - it's unlikely that I would want to fuck a particular person badly enough to fuck up the friendship. In the OP scenario - I don't see an issue with pursuing/fucking the girl. They've been divorced for a year, he's a friend of a friend. If he has an issue with her fucking other guys, he probably should have been a better husband. If it came up socially with blow back, I would expect my actual friends to understand that he was a cheating douche bag, she's smoking fucking hot, they've been divorced for a year, and he's some guy that you know, not actually a friend. Ruling out every person who's ever been involved with anyone I know would lead to a lot of jerking off using my own tears as lubricant. Fuck that noise. If they didn't get that, I'd probably want new friends. I probably wouldn't raise the topic of her ex at all in conversation with her though. There aren't many situations where I'd raise the topic of someone's ex in conversation, while I was trying to fuck them. I would just approach conversation with the girl like I had no involvement with or knowledge of her ex at all. If she raised the topic - I would just assume she already knows everything she needs to know and there's no reason for me to fill her in on the details of her own relationship. If she clearly doesn't know any of the details or if she asked outright - I'd have to play that by ear and respond depending on what other information was available. IE if she thought she'd been in a 100% monogamous relationship and had been having unprotected sex with him, and hadn't had an STI test after the relationship ended? I'd probably need to fill her in on the details. But it'd really depend on the details of our interaction.
There is no given rule for these situations. This is obvious common sense, but you need to view the scenario in context. If you want to hook up with a friend's ex, or a person they have expressed interest in, you should judge how you think they will react, and if you are OK with that reaction. You also should anticipate how others will react. Put more simply, be ready to face the consequences of your actions, and think about what those consequences might be before you unzip. I always try to avoid friend's exes and girls they have a crush on and such, because there are lots of girls I can sleep with so why bother with one that will cause grief elsewhere. There are exceptions, of course. If I know it won't be a big deal I'll still go for it even if it's a good friend's ex. But if I know a friend whose feelings I value will be hurt by it, I stay away. Alt Focus - actually calling "dibs" on a girl is nonsense. If you don't lock the shit down, you lost it, no one else stole it. If you don't command enough respect among your friends for them to stay away from a girl about whom you've expressed a clear intention, you either have shitty friends or they recognize that you're a big old pussy.
Personally, my general thoughts are if you're going to do it just to get your dick wet, don't. If you think there is a shot at something more meaningful, then go ahead. Like others have said, though, think through the potential down side to this, and be prepared for it should shit hit the fan. Hope for the best, plan for the worst.
If this guy is genuinely your friend, I would say ask. If he is just some random dude with whom your social circles overlap then play ball.