I've almost finished a quilt for my bff's BIL who has a terminal illness.
I am only acquainted with them so I don't want it to come off like some pity thing. even though it is. I have no idea what to say.

help

Just tell him the quilt was made with love - so that he will always be wrapped in love.

And depending what type of illness and treatments he has going on he made need it to keep warm ?

Or you can pass it on to your BFF to give to him and just say its from a friend

"A quilt to comfort you"? I think simple is best in this case

I would just try to be as honest as possible. Say that you know things are rough and just wanted to help brighten his day with the gift of comfort. Or send it with your bff and pass the explaining on to someone who knows them better.

A gift of comfort is perfect. Thank you
:)