Ya, have her quit laughing so hard underwater....
It is likely the strap not being tight enough, the new cloth straps do not have much bunji effect and to not push the mask against your face snug it up then take it off and tighten it a little more. she may also be breathing out through her nose and pushing the mask away. but I have found that a mask that fits me fine otherwise will leak if the strap is not tight enough also sometimes the extra bit of strap folds back and gets under the seal.
It is likely the strap not being tight enough, the new cloth straps do not have much bunji effect and to not push the mask against your face snug it up then take it off and tighten it a little more. she may also be breathing out through her nose and pushing the mask away. but I have found that a mask that fits me fine otherwise will leak if the strap is not tight enough also sometimes the extra bit of strap folds back and gets under the seal.