The rest of the evening brought her little amusement. She was teased by Mr. Collins, who continued most perseveringly by her side, and though he could not prevail with her to dance with him again, put it out of her power to dance with others. In vain did she entreat him to stand up with somebody else, and offer to introduce him to any young lady in the room. He assured her that, as to dancing, he was perfectly indifferent to it; that his chief object was, by delicate attentions, to recommend himself to her, and that he should therefore make a point of remaining close to her the whole evening. There was no arguing upon such a project. She owed her greatest relief to her friend Miss Lucas, who often joined them, and good-naturedly engaged Mr. Collins's conversation to herself. (Chapter 18)

Lizzy points out Mr. Collins to Charlotte at the ball so I think Charlotte and Mr. Collins must be first introduced that evening. Lizzy had not seen Charlotte for a week so it seems reasonable this was the first meeting between future spouses. It must be obvious to Charlotte that Mr. Collins is bent on Lizzy and it must be equally obvious that Lizzy will not have him. Lizzy’s greatest relief at the ball from Mr. Collins’ persevering attentions is that Charlotte good-naturedly engaged his conversation to her. Is Charlotte just being a good friend or is she also acting practically in her own best interest. I do not think Charlotte would consider Mr. Collins if she thought Lizzy would have him but that is not a part of the equation. Is Charlotte laying the ground work for a conquest of Mr. Collins at the ball? ;D