Running the dp/flex I would simply burn the flex and substitute her in for the injured player. If I lose two im going to hear about it from everyone who has been previously happy with their daughter's playing time under my method. Lol
You cannot simply burn the flex and use her as a sub. The only spot the flex can bat in the lineup is in the DP position. You would still be able to field a full defensive lineup by using the DP to play in the vacant fielding spot, but if the batter lost was anyone other than the DP you will have a vacant batting spot in the lineup and an automatic out every time that batter is due up.