Is there a 3rd party that could talk this through with her (aunt, cousin, or close family friend). They may be a bit more removed so could talk with less emotion.
If all else fails then you may just have to support her (or at least pretend to) with a few musts in place- you will have to negotiate on these. but I would aim for-
1. If you are to respect her choice with no bias about his age then she must do the same - so same rules for this relationship as for any with someone her own age- which for me (with my dd would be home at night until she is 18). To do this you will need to be able to have him in the house as you would with any other boyfriend.
2. She should understand, if this is for the long term, what she is getting into and if you accept the age diff and support her (grit your teeth when you tell her that) she should also accept the age diff and prepared for a life when he will retire and not be the bread winner, so collage would be a good idea as she will probably in the future need to surport both of them. Also as he as kids of his own his income will not be all theirs. You may be able to get his support of this as well (if he has any decency in him- but with that age gap I'm not sure he has)
3 I would also think about inviting him and one of her own age friends on your hols- 1 week with you for each. (yes you we prob ruin your holiday but - you may get her to see the age gap more clearly- I would get them tickets to US/IOA and for a very loud & late night in between at city walk.)- Which one is the most fun to be with may then become very apparant.
If all else fails - then you'll just have to be there for when it all comes crashing down- parents raw end of the deal I'm afraid. Let her know that it won't be a told you so situation and that your there for her regardless and that would go for any relationship not just with this man. (again show no bias)
Good luck
I didn't mean straight away, but thoses ideas were fall back if all else fails.
Just another thought, does this man have any family, sisters, brothers, cousins (or even children) that you could approach for their help. I'm am sure that even his own family would also be very upset about him seeing your dd.
Also a bit worried that your dd doesn't want you to contact social services, does she have a reason for this. I thought you had already contacted them, I would do this ASAP as they must have experience re this kind of thing. He may have done something like this before and be on there files.
Good luck
But rememeber,do not cut her off- she needs to be able to come back when it all comes crashing down.
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God, what an awful situation to be in. I would be beside myself with worry and anger. Unfortunately there is very little you can do as she is 16. I think you should keep talking and 'tolerate' the situation rather than accept it; unless of course you find he has any sort of record!
They will face a lot of prejudice and this might make your DD think twice about this relationship once she goes 'public'. He will inevitably lose family and friends over this, which might make him think twice about the relationship! She may well grow out of him too - a clandestine, forbidden relationship is so much more exciting, so perhaps once the dust settles, it will seem a little bit mundane.
I would keep talking and listening, but I would explain for now that it has been too much of a shock for you to start accepting him and visiting his home. If he really loves her, he would be happy for her to live at home, finish her education and leave home at 18. Why not suggest this as a compromise? If they are still together then, you might have to give the relationship your blessing, or risk losing her.
Relationships started at 16 rarely last into adulthood. The odds are stacked against them. I really hope it all works out, Debs.