Y’all have put many great suggestions out there - from a parent’s point of view, which is great for the OP.
But I’m here to give the daughter’s POV. This is gonna get LJish and a bit TLDR, so you can just ignore me if you want.
A bit of background - I spent most of my teen years online, because (unlike your daughter) I didn’t have a great social circle or lots of extra-curriculars. At 15 I was working, at 16 - which it sounds like your daughter nearly is - I had left school and was working full-time in the adult world and at 17 I’d moved halfway across the country with some guy my family didn’t know. So this information is relevant from my experience, which is of an “old” 15 year old as my mother called me. If your daughter is a bit more naivé or a “younger” 15 year old then it’s probably not going to apply.
To start, yeah there’s a chance that the guy she’s talking to is some weirdo grooming her for nefarious purposes. But there’s an equally good chance he’s not. During my online years, I had about four or five online boyfriends including the man I eventually married. My (now) husband is three years older than me, one was four or five years older than me. The rest were all about my age, which at the various times made them anywhere between about fourteen and eighteen. I established this in various ways. Talking on the phone, video chat online, talking to family and getting friends to go and meet them by proxy when I couldn’t meet them myself. I only found one person who ever lied directly about who they were online, and that was someone who pretended to be a female friend, who then was “murdered”.
Because this was before online grooming became a big issue, I didn’t have the level of restrictions you’ve given your daughter, so I did give out our home phone number and our address. And yes, fourteen and sixteen year old guys do buy and send jewellery. In my case, it was nice but clearly Wal-Mart quality jewellery. And that’s the first thing I’d be looking at. Has he sent her a trashy silver-plated cubic diamondium necklace? Or has he really sent her a white gold and pure diamond necklace handcrafted by christian dior? One is not like the other, the latter is not what you could reasonably expect a schoolkid to afford, and would set off warning bells. The former could just be he works a paper route and spent $15 buying something “nice” for his GF.
Also, the “kinky” thing? At fourteen, I was a rampant cybersex bunny. I’d already done the physical deed with my ex, so with my online boyfriends it was only logical that because we couldn’t express our love physically we’d cyber like mad things. So yeah, mid-teen boys will talk about “kinky” dreams and press to hear about the fantasies the girl’s had. Again, slightly creepy factor when you understand that he is masturbating to this stuff but not necessarily an indication that he’s a paedophile looking to steal your daughter away.
The marriage proposal. I’m glad you’ve established this is an online wedding. But again, I received proposals from two of my cyber boys before I met hubby. Because we thought our love was TRUE and ABIDING and would NEVER DIE so we’d be TOGETHER FOREVER!!!one!!!eleventy. These went out the window as soon as we broke up. The other thing to remember is that if he is a real boy, there’s every chance he’s going to find another girl either at home or closer to home. Computer love just ain’t the same as necking in the back seat when you’re fourteen or fifteen.
Forbidding your daughter will achieve nothing. The other suggestions made here have been the spot-on ones from the daughter POV in my opinion. Sit your daughter down and explain why these rules have been made, and why you’re upset that she broke them (ie for her personal safety). She will roll her eyes and think you’re just oppressing her, but it’s something that will come back to her when she’s older and this is all over. Explain to her why you’re upset that she’s broken your trust, and whilst you’re not going to forbid her to “see” this boy you need to know about him, his name, his address, see a photo, talk to him in a video chat and/or his parents on the phone - again, to make sure he’s on the up and up and won’t try to hurt her.
Personally, I’d even go so far as to give tacit approval to her meeting him in person - if you are there to meet him too. If he’s really fourteen, the meeting will probably never materialise. But if you give the appearance of more or less approving of things, she’s more likely to be open to saying “Chris said he’ll be visiting our town/somewhere nearby can we go and meet him mom?” rather than sneaking out one day. The other thing is that if he’s really fourteen and she says “Mom will be coming to meet you with me” then he’ll probably go “Uh, cool”. If his response is any different (and you’ve further been unable to establish his real age), then you’ve got a better indicator that he’s maybe not on the level and can then make the decision about whether to take this further with the authorities.
But this is just my 2c as a kid of the 'net age, and I don’t envy you this task. I’m conflicted myself. I don’t have kids yet, but I dread if we ever do. Because my times online, whilst sometimes filled with conflict and stupid people, were pretty great. I took some risks that would have some parents clutching their hearts and fainting, but I met some great people and even my husband out of it. But then I also know that online predation is becoming a greater and greater threat and I would do anything to stop my (potential) kids from being victimised by it.
Good luck.