What did Mrs. Higgins tell her son about his behavior and experiment with Eliza ?

Mrs. Higgins reprimands her son for treating Eliza like property. 

MRS. HIGGINS. Well, of course he did. What right have you to go to the police and give the girl's name as if she were a thief, or a lost umbrella, or something? Really! [She sits down again, deeply vexed].

She also lets him know that she disapproves of the way the men have taken advantage of Eliza's affectionate nature and desire to please.

MRS. HIGGINS. Just so. She had become attached to you both. She worked very hard for you, Henry! I dont think you quite realize what anything in the nature of brain work means to a girl like that. Well, it seems that when the great day of trial came, and she did this wonderful thing for you without making a single mistake, you two sat there and never said a word to her, but talked together of how glad you were that it was all over and how you had been bored with the whole thing. And then you were surprised because she threw your slippers at you! I should have thrown the fire-irons at you.

She also blames Henry for Eliza's spoiled behavior;

MRS. HIGGINS. I'm afraid youve spoiled that girl, Henry. But never mind, dear: I'll buy you the tie and gloves.