Just a few facts needed here:
First, Oxytocin can be given about any time. We use it day before breeding if a mare's uterus is a little big or we have been treating for an infection. It causes uterine contractions and helps expel fluids and shrinks the uterus, even in non-foaling mares. It is appropriate to give in this situation.
Second, Founder is your biggest possible problem if she did, indeed, retain part of her placenta. Founder following a retained placenta is usually a very catastrophic founder. The hotter the weather, the faster this happens.
If she did retain part of her placenta, she will probably have drainage and a discharge by now.
Thirdly, Antibiotics like Penicillin are pretty useless. To deal with a uterine infection from a retained placenta, the antibiotics need to be infused with a large volume of saline solution directly into the uterus. Shots are useless.
Most big breeding farms are set up to do all of these things 'in house'. Otherwise, you need to get mare to an equine Vet or have him meet you at a breeding farm.
All breeding farms have stocks. We let other people use ours and let other Vets come here to doctor their horses. Most breeding farms will let you do this.