Thank you for your question and patience, I’m Tom and I’ll try to help you.
The eligibility criteria for a visitor's visa is as follow (as I am sure you know), is that you need to show that:-
· You want to visit the UK for no more than six months;
· You intend to leave the UK at the end of your visit;
· You have enough money to support yourself during your stay in the UK without working or needing help from public funds
You have to try and show them that she has continuing obligations in her home country which will continue after the visit and therefore convince them that she is not a risk of overstay because she at present has compelling family and social/work ties to her home country which mean that she will not attempt to secure a permanent stay here whilst she is visiting. Things like a letter from her employer stating she has a job (if he does) and is expected to return to continue that job, return flight tickets, evidence of continuing accommodation (eg. tenancy agreement of evidence of ownership of a property), bank statements showing the money she has available, letters from relatives/friends confirming any obligations she has to them.
If you are based in the UK and will assist her during the intended stay you should go to a local solicitor to draft a sponsorship declaration in which you state the terms of the visit, that you are to return at the end of it, that although you are in a relationship with her you both know that she MUST leave at the end of the visit because it would harm the chances of any future application made to secure leave to remain on the basis of that relationship, that you shall accommodate you and shall pay the costs of any unforeseen expenses that should occur during his stay so that nether of you will need to access public funds.
This should cost around £50+vat and you can find immigration solicitors via:-
My goal is to provide you with a good service. If you feel you have received anything less, please reply back as I am happy to address follow-up issues specifically relating to your question.