They could only sue you IF you did not have any reasonable grounds for accusing them of assault but instead did so for improper or malicious reasons, such as to harm them, make them spend money, get back at them for something, try to extort them into doing or paying something (e.g. "unless you pay me the money you owe me, I'll have you arrested for assault"), etc. The fact that someone was not found guilty, or the fact that the witness or victim was wrong, does not give rise to a legal claim--only misuse of the legal system does.