I want to tell my brother his girlfriend looks like a different person than on Instagram
That's strange, can you tell me exactly what you said?
She's ugly and I wouldn't even want her as a friend.
You shouldn't tell him such rude things about his girlfriend. He would resent you and think you are shallow.
My brother already told me that she looks like a ladyboy.
This is quite the strange take on your brother's girlfriend and it is strange that he believes she is ugly too. You shouldn't be saying rude things about your brother's own partner because if he disagrees it could start a huge fight.