Oh, my! these parents are everywhere! One thing that they need to remember is that the umpires/referees remember the loud, rude, back talking parents. Not just the comments to the officials, but about the coach, teammates, or the other team. Unfortunately, this means that they start watching very closely for errors.
Another thing they need to remember, what they say is usually heard at home as well, so her son is probably hearing this and it colors his view of the team. Instead of going out and having fun, he will most likely be trying to deflect his own errors (and they all have errors, even the best of them) on somebody else..."I missed it, but Joey threw it wrong", "Bobby may have made the basket, but I was in the 3-point range and he should have just passed it to me", etc. It can make the whole team tense and take the fun out of it for everyone.
I agree that she needs to be talked to, nipped in the bud. The "sorry this isn't working for you, maybe the other team is a better fit" speech may be the right approach. It may be your husband that has to give her the speech, though, to make it more official.