From the Xcel CoP vault deductions for gold, platinum and diamond:" There is no deduction if the coach, when standing between the board and the vault apparatus, makes
contact with the safety zone mat or inadvertently touches the vault apparatus". So as long as he doesn't touch her, i think there won't be a deduction.
We have seen that happen if the mat had been slipping out when the gymnasts land. The coach's weight kept the mat from slipping, so the gymnast didn't have to worry about an additional balance issue (kinda hard to land with the mat moving under you).