Firstly I’d suggest not spending time in their company, cos this person sounds like a total fatophobic dickhole.
If they can’t see that Finn is into Rae because of her personality, her sense of humour, her taste in music, etc. from what they’ve seen on the show, nothing you can say can probably convince them!
If this person starts talking shit again, you should just tell them that Finn likes Rae because he’s a better person than they are.