Who ultimately decides questions like this. One side says the canons of the Church forbids prayer with Roman Catholics; the other side says it is allowed.
Behind every canon is some core "truth", an unwavering principle which, then, gets applied to a particular situation in order to show how to resolve it. With regard to "prayer with heretics", I think what the canon is trying to prevent is any sort of "mixing" that would corrupt Orthodox faith. As Orthodox, we believe that the way we pray is the way we believe, and what and how we believe is reflected in what and how we pray. And when we pray in common with others, we, in a sense, become "one mouth and one heart" with our fellows. When we join in prayer with non-Orthodox, it can "corrupt" Orthodoxy: it doesn't have to if the Orthodox people involved are strong enough in their faith, but it can and does happen.
This is what I believe the canon is really trying to address. Now, that means that participating in other Churches' sacraments is definitely a no-no, for to do that would imply that you are one in faith with them. But what about worshiping with them? I believe if someone makes a habit of prayerfully participating in another Church's worship services, that'll eventually have an effect on your faith, because any attempt at actual prayer will involve the body, the mind, and the heart working together. For that reason, I think it's better to avoid it (I wouldn't say it's a prohibition on attending, per se, but there's a difference between respectfully attending and participating). What about praying with individuals? Opportunities for this can present themselves in mixed families, among friends, or in other circumstances. I've seen plenty of situations where even this can lead to the Orthodox person(s) getting weaker in the faith, or thinking that everything is the same, or even getting more and more interested in the other faith. At the same time, it would be pretty bad, perhaps unnecessarily bad, to alienate your family, in-laws, whatever by not joining in the prayer before meals or something like that. So personally, I think you need to use some common sense. When the prayer is a set formula (e.g., the Lord's Prayer) or is extemporaneously offered by someone you consider trustworthy enough not to go full-blown heretical, it's probably OK to join in. If you don't know what you're getting yourself into and can't find out in advance, or if you know it's going to be an issue, it's probably best to find a way to avoid it.
Ultimately, it is the bishop who applies the canons for those under his jurisdiction, and typically his priests will know his interpretation of things like this. So I think "ask your priest" should suffice in most cases where one is not sure what to do, and if he's in any doubt, he'll know how to clear that up.