I don't think it's ever wrong to register for gifts or maintain a wish list. That goes for people who are married, engaged, single or anything in between. What's rude is what you do with the registry. If you use it as a wish list and only refer people to if they ask for gift ideas, that's fine. If you send the registry link out to all and sundry, that's tantamount to screaming "give me gifts - and here's what to give me!!!"
So if you only know because your BFF's son mentioned it to her in passing and then she mentioned it to you, then I don't think it's anything to get worked up about. If they are sending out marriage announcements and including registry info, then that's rude. But still one of those things you just privately roll your eyes about and move on, letting it inform your opinion of the person (and potentially influencing whether or not you send a gift/what gift). But I don't think that it's actionable rudeness.