I'd have to say that if she's asking she is not prepared to own or even work with a stallion. I worked with well over 20 stallions before I even thought of owning my own. I've worked with TB, Appy, QH, WB, STB, Andalusian, and draft stallions.
Some stallions if regularly turned out with other stallions or geldings or even bred mares can continue to be turned out with other horses, however simply by nature they are more likely to be territorial or aggressive. I own a stallion, two mares, two female mini donks and a female pygmy goat. The goat can go in the field and share hay with the stallion, commonly does are in the field with him and the donkeys spend time in the field adjacent to him. However, he just recently took offense to a 200+ pd mastiff while staying in a field that wasn't even his. I don't think he would have done this had he been haltered and supervised, but when you turn out in a field you aren't going to be standing there holding them right?
What happens if/when they he gets out? Any damage including unwanted breedings are the responisibility of the stallion owner. Why is she keeping him a stallion? Is he something exceptional? Many stallions would make lovely geldings. Also at four he may not yet be breeding. Once a stallion is a working, breeding stallion he may change.
Not even fully knowing the situation I don't think it's a good idea. People think I'm very strict. My stallion may NOT speak while being handled or ridden, no calling or nickering. Rarely does he get handfed treats. He will get carrots or apples on a bed of hay or in his feed bucket. Your stallion must respect you and you must respect your stallion.