My stays for each of my hip replacements was 4 days but I was not 87 yrs old either. In the US, sometimes instead of dismissing hip patients to go directly home, surgeons transfer them to a rehab unit first. This might be the case for a patient who lived alone or had co-morbidities or little social support. In a planned hip replacement, this issue is usually considered before the surgery so that the patient and his caregivers can plan for his recovery, either at home or in rehab. Was this talked about at all? Were you or any other family member or friend involved in her pre-op planning?
IMHO, if she is dismissed to go directly home, she and her husband will need some kind of assistance. If her surgical experience is anything like mine, & if she goes home instead of to rehab, she will have visiting nurse and physical therapist visits for at least a couple weeks. This is what I had. After those visits end, she might need at minimum: house cleaning help, someone to run errands, buy groceries, pick up meds, & drive her when she needs it. Early on, she and her husband may need meal preparation assistance. If her community has meals on wheels delivery, this could be a solution. Transportation help can come from local or regional on-demand mini-bus service (my own county operates such a service to take people who can't drive to doctor appts, shopping, etc.)Help with personal care such as bathing is a part of medicare coverage - charged on a sliding scale based on income. My father had this kind of help.
Usually the hospital works all this out at discharge. They usually have a social worker based in the hospital who does this kind of stuff.
Don't feel guilty if it isn't possible for you to stay with them for a long period of time. You may be able to handle some things on the phone in between shorter visits. Use your own judgment and your knowledge of your parents' abilities to make your decision. You can begin by talking to her surgeon or primary care physician about her discharge plans and her needs.