From what I’ve read generally anything that is safe for human and horse consumption plant-wise is also safe for rabbits, although I’m not sure how it will fair for the rabbit on a long term, I don’t want to give bad advice but I suggest buying a 50lbs box of Oxbow timothy hay for $53 on Chewy, you’ll get it in a day or two. Grass/hay is essential to rabbits since it helps wear down their teeth and get the fiber they need.

Ask him what kind of hay he feeds his horses.

If he feeds meadow (grass), or oaten, you will be fine. If he feeds wheaten or lucerne (alfalfa) then no. If he does feed a meadow mix just ask if it was grown with any specific grasses or weeds to be safe – sometimes they are. But usually it is fine.

My rabbits have lived healthily and happily off my horse hay for years. Provided it is actually good hay it is fine for rabbits.

“Rabbit hay” at supermarkets and pet stores is sometimes grown with extra nutrients, hence the price tag, but that’s really only needed if your rabbit is underweight or unwell. A lot of the time I have seen horse hay sold as individual wrapped biscuits for over inflated prices.

If you have somewhere to keep a bale of hay, get a whole bale. But if you can get hay from your dad in the meantime until you can get a bale, then go for it. 1 biscuit of hay should last you a bit anyway, unless you use it as litter.