As a self confessed film obsessive I have seen all of the above mentioned so:
Warhorse – Not bad but not great. It’s classic Spielberg, a popcorn movie which knowingly attempts to tug at your heartstrings. It was a bit much for me though. You’ll either enjoy it for what it is or vomit and accuse it of being too manipulative.
Sherlock 2: Same as the first one. Decent flick, nothing amazing but no worse or no better than number one. Just good fun.
The Grey: Looks beautiful, sounds beautiful and has some ‘moments’. A visual feast but no great depth.
The Artist: Perhaps not quite as overwhelming as some reviews have made it out to be but it is a great movie. It’s different without being challenging. I defy anyone not to enjoy it regardless of your standpoint on film.
The Descendants: Wonderful film. Clooney is superb. Heartwarming and well crafted.
My pick would be The Descendants as I’m sure both you and your mum would enjoy it.