Looks like this thread has gathered together all the vile ignorant people who think abused women deserve it, brought it on themselves, chose it, etc etc. The sheer number of posters who can't comprehend why the op describes her friend as vulnerable is telling.
No woman deserves to be targeted by an abusive man and "common sense" or "stupidity" have little to do with it. Abuse happens to women from all backgrounds, although some indeed are at greater risk. The person at fault is the abusive man.
Whether it's your garden variety abusive man living in the UK and targeting women in the UK, or one living in a different country targeting women in the UK with his eye on a visa.
The former can play a much shorter game than the latter who needs to manoeuvre his way to a visa and settled status, but ultimately they both deploy the same manipulation and isolation of their target, and both have the same impact on their target. Adult women can be groomed just as much as children.
The defensiveness of your friend when challenged, op, is the same as is frequently described on here by posters worried about a friend or relative being sucked into an abusive relationship. You stand the best chance of helping her if you don't do anything that leads to he becoming isolated and cut off from her entire support network. There is advice on the Women's Aid website that may help - don't tell her to leave him, don't suggest she's stupid for believing he loves her, don't badmouth him. Ask questions as opportunities arise in natural conversation that give her opportunities to voice doubts or start thinking through concerns privately, but without making her feel judged or attacked. She needs to know if she walks away from him that she won't be roundly mocked and condemned for being stupid.
It's common for women who've been raised in abusive households to end up being abused as adults and going from one abusive relationship to the next. Not because they are stupid or deserve it or want to be abused, but because abuse has been normalised for them, because nobody has ever taught them what a healthy relationship even looks like for them to know life could be different, because nobody has ever taught them what abuse is or how to spot warning signs, and because they think it's normal to be abused and are used to it and used to having to put up with how awful it makes them feel.
How many times do we see posters on here who go from one abusive relationship into another one because the new man started off comparatively lovely next to the ex and the poster had never been given the tools or knowledge to use spot warning signs of abuse so just decided the new guy wasn't abusive because compared to the by then severely abusive ex in the end stage, the new abusive man in the love bombing stage seemed like a dream come true.
If you have an ounce of knowledge about the dynamics of abuse and compassion for your fellow human beings it is not difficult to understand this.
The only difference with your friend's current situation is that there is so much more at stake. It will most likely be years before his true intent is revealed - he can't risk her realising before he has what he wants, the same as Uk-based abusive men keep up the pretence until after the woman is pregnant or has married him.
It's sad there are so many nasty, ignorant people on this thread condemning women in difficult situations that these posters have demonstrated they understand nothing about.