UK has double taxation agreements with many Europen & other countries, including some African ones. Your friend should look at the HM Revenue & Customs website to see if his country is part of that agreement. At least she may be able to threaten him with HMRC looking into tax he owes them, especially if he has a part time job as well, and at best it may shed light on what else she can do.

Also if your firend has parted with so much money then she should be able to claim back tax paid on the interest of that amount.

Cultural values about men and women's roles in a relationship should be taken on board before relationships are entered into. All men can charm when they want something, All can dismiss when they choose to ignore that same something as a debt, morally or otherwise.