Being disabled mentally / having development issues (resulting from brain trauma, in this case) is a wide spectrum. Meaning, no two people are 100% alike with their disabilities, when it comes to disabilities such as this. For example, I was born with quite a bit of white brain matter (that means blank spots in the brain) at birth, due to being 2.5 months premature. The doctors told my mother that I would most likely grow up to be extremely slow, with a low IQ, and they tried to kindly tell her that I'd probably be... stupid, essentially. Well, I have an IQ of 140 and was reading and writing by a very early age, and, I went to a good college that was fully funded to me by grants due to having an extremely high GPA. Now, that being said, only the people who are living with mental issues / development issues / brain trauma can know how much they can or cannot do something. Someone with brain trauma and development issues may very well be too stunted to fully consent, but, someone else also in the same situation may very well be able of such things. We can't fully know because we aren't in the minds of the people who have those issues. It's a very personal thing and brain trauma and how it affects someone can be a wide spectrum that we may not fully be able to grasp. However, I will say that her ability to love, to be in love, and to want to get married should be a testament of her sexuality and how comfortable she'd be with consenting. Being physical with someone who you love is just another expression of love - it's a physical expression of love. So, if she's in love with him, wants to marry him, etc. then she already feels that love inside her heart. Any physical acts between them would just be a physical manifestation of their love, so, that's one way to look at it.