If my husband daughter gets her GED at 16 , does he still have to pay child support until she's 18?

Depends on what the support order says.

Yes because she is still a minor child. Chid support stops at 18 and graduated High School (in some states 21) or at emancipation. Having the GED could qualify as graduating High School but then the child remains under 18 and not able to sign contracts or live independently on their own.

In most cases, yes it will continue, it is surprising that they were allowed to do this at 16.