loveless marriage - long story
Is that a euphemism for abusive?
Because the woman you're describing sounds like women tend to be after getting out of an abusive, controlling relationship - extremely vulnerable and at risk of being targeted by other, differently abusive men who target and groom them.
Grooming isn't something you can undo by telling her to do what you want. You need to bring her around to your view gently and get her to realise for herself - ask questions to get her thinking and give her opportunities to raise doubts or niggles (without you immediately flying off the handle or jumping on it to tell her you were right).
Otherwise she'll dig her heels in and carry on to prove you wrong.
If she insists on going, have safety plans with her. Print off the FCO travel advice, and anything else appropriate then chat it through - calmly, non-judgementally. Give her information and help her to feel in control of her decisions.
Help her set up a travel money card (for travel safety reasons) if she doesn't have one and get her to leave debit/credit cards at home. (So if it turns exploitative and he suddenly has an emergency he needs her to help with...)
Remember, if she goes and then starts to have doubts you want her to feel able to call you and ask for your help. You don't want her to feel too ashamed or embarrassed or afraid to call you or tell you because she thinks you'll just say "I told you so".
livingtheturkishdream.com/2013/09/15/holidayflingorrealthing/
You could use something like this for the basis of a calm conversation - it would show you're open to accepting she's found happiness and you're there to support her however things go. And it will allow you to plant seeds of what warning signs she needs to respond to (don't tell her "if this happens, you must..., just talk through the red flags and cases where women have been exploited. Mention them as a light hearted joke ("I'm sure he's not going to be one of those who suddenly has a financial crisis as soon as you go to visit his family! Ho Ho") or as questions. Not instructions.)
Do some basic travel safety planning with her if she's travelling alone.
Worst case scenario, figure out a backup plan of what you can do to help her if it all goes wrong and she needs your help. It will give you a little measure of peace of mind if you have a plan prepared in advance. Copy passports, contact details, addresses, embassy location and contact details, etc.