Well done to your friend for being prepared to "stick it out" and persevere to settle Henry into his new home - he is a very lucky cat!

I havent really got any experience with abused cats, I have however had a couple of feral kittens in my house for short periods (usually only an overnight whilst I make arrangements with the SPCA). What I have found works well is to make sure they have somewhere they can "hide" where they feel safe - some cats like a cardboard box with a hole cut in the side, and a blanket inside, others prefer something less closed in - like a blanket in a corner of the room, next to a sofa or something. Some cats are happier when they can climb up high - do they have a closet or something where they could give him his own special place? He may also sttle in better if he does not have the run of the entire house - maybe resrticting him to one room which he can get used to will not be so overwhelming for him, and once he has settled down he can explore the rest of the house at his own pace

I suppose the only real cure for this is lots of patience and love. But I am sure if your friend perseveres she will be rewarded with a very loyal and loving pet at the end of it all. Best of Luck!