Thanks for your question. I will try to help.
Unfortunately, because by the time she has leaves she will have stayed for 6 months she will have to wait for a further 6 months once she has returned to Brazil before she can apply for a visit visa again
This is because no visitor is permitted to spend more than 6 months in any 12 months period in the UK and so she must wait until she clocks a further 6 months outside of the UK.
Unfortunately, there is not visa that she can apply for to extend her stay for this reason. The only possible visa would be if she were suffering from acute health problems and to make her travel would be prejudicial to her health, but I assume this is not applicable to you.
She is free to apply for a further visit visa after she has returned to Brazil for 6 months, but the entry clearance officials will obviously be a little concerned with the amount of time she is spending in the UK and also that her boyfriend is in the UK. They may view here as an increased risk of an overstay for this reason and so it is essential that you demonstrate as much evidence as you can of her compelling family and social ties to Brazil to show them that she is not a risk of an overstay.
My goal is to provide you with a good service. If you feel you have received anything less, please reply back. Please remember to leave positive feedback using the stars at the top of the page.