If someone reserves a card from the family group but doesn't complete it, can the person who initially reserved it get it back?
I am just another 'lowly' User/Patron ...
[ And, I happen to be a Member of the Church ... ]
Just in passing ...
In the situation/circumstance, that you have ...
Short Answer: NOT until, the "Reservation", by the OTHER User/Patron, has EXPIRED.
The User/Patron, who ORIGIGINAL "Shared" with the "Family Group", CAN "Unshare" from the "Family Group" (ie. Take back the "Temple" Work); but, NOT until, the "Reservation", by the OTHER User/Patron, has EXPIRED.
I hope, that this may help/assist, somewhat.
Here is a "Knowledge Article", in 'FamilySearch':
Where, it states, among other things;
The user who shared ordinance with a family group can unshare ordinances from a family group as long as no other group members reserved them.
I hope, that this may also help/assist, somewhat.