Villa have been poor for a number of season, and it isn't down to one player/manager, but the team as a collective. The manager has the biggest piece because they are the ones who pick the team and the tactics, but once they walk on the pitch it's down to the players. I don't think booing either is productive to the team.
The difference I do see in booing a manger over a players is a player is on the field, and has to perform for the team. The manager is on the sidelines, and has to shout commands at the players. Much of what the manager says goes unheard, and players normally do what was set out before the game. The player is the center of attention, and if he is booed every time he touches the ball, this is going to put more pressure on them, which will bring more mistakes.
Westwood hasn't been a superstar, but he also hasn't been the worst player over the last five years. He has been to do a very boring job, and if it goes wrong people get on his back. This is the same reason defenders get a lot more stick than forwards do. Forwards can get away with a lot more because they score goals, but I'm pretty sure, if you were to add them up, our forwards make more mistakes than Westwood. Grealish gets away with stuff because he makes and assist or scores a goal once in a while, but he gives the ball away so much, and most of his dribbles end up with lose of possession.
This is a time to support everyone as we hope to continue to turn things around. Players need our support more than ever right now. and booing individuals isn't going to help.