If my daughter’s husband said that the health insurance cost is deducted from the child support money, is this true?

Mississippi, like most states, has an actual formula for calculating child support.  It's found at this statute: Miss. Code Ann. § 43-19-101, which you can view on your state's website.  They will take his income minus qualified expenses, and then order him to pay a percentage of that amount as child support.  Health insurance is not deducted from the calculation of that amount. 

However, as a secondary support order, the judge can order, in addition to regular child support, that her husband pay all, some, or half of the premiums for health insurance.   If a judge decides that the parties should split the premiums fifty-fifty, then they could agree to an offset against the child support to prevent one spouse from having to send a check to the other.  For example, if he was ordered to pay $250 in child support and was ordered to obtain coverage through his employment which was $100.00 per month, then your daughter may want to take $50 less in child support each month instead of sending a return check for the insurance--- but only if she is ordered to contribute.  Often, the courts will require one party to pay the insurance or will order both parties to apply for state or federal aid to insure coverage for the child.

Whether or not your daughter's husband's statement is true or not about insurance costs being deducted from the child support money can only be ascertained by an accounting of payments to the insurance policy with a recap by the husband showing where the payments have been made to. You need a formal accounting with back up documentation to prove what is claimed or not.