This is generally how it works. The procedures are pretty similar through out the States. The relative will call the Dept. of Social Services and tell the person answering the phone they need to report suspected abuse of an elderly person. They will be tranferred to(or will leave a message for) an Adult Protective Services intake worker. The intake worker will want as much and as specific information as possible. The name of the elderly person, the name of the alleged abuser, the specific nature of the reporter's concerns and as many details and specific examples and incidents as is possible. Although your relative can make an anonymous complaint, the complaint will carry much more weight (and will be much easier to successfully investigate) if s/he will provide contact information. To the best of my knowledge, the name of the reporter is confidential in all 50 States, but the reporter may want to emphasize that they do not want their name revealed as the originator of the complaint if concerned it will make their own life much more difficult if the neighbors figure out where the info. came from. The intake worker will also ask for the names of others who may have knowledge of the abusive behaviors. It will be helpful if the reporter is prepared to share any knowledge or impressions they may have regarding the elderly person's physical frailty and whether or not they believe the individual suffers from cognitive impairments, such as dementia. Do they know or suspect that the elderly person, the care giver, and/or the blood relative have a hx. of mental illness or currently active substance abuse? Your relative should be prepared to share both what they know and what they suspect or think, and to distinguish clearly between the two. If the relative otherwise has no ax to grind with anyone in the neighboring household, they should make that very clear.
My understanding from your earlier posts is the elderly individual lives with a blood relative and the relative's partner. The partner is the primary caregiver during the day and is the alleged abuser. The blood relative is apparently unaware of at least some of what occurs during the day and there is some indication that the partner takes pains to conceal some of their actions. If this is the case, this should be made explicit to the intake worker.
Remind your relative that s/he is not filing a criminal complaint, and the alleged abuser is not likely to be approached from that standpoint. Assuming a reasonably competent and adequately trained and supervised Adult Services Unit, they will investigate the situation with an eye to the needs of the family unit, will be very aware of the stresses that can be involved with caring for a physically or mentally incapacitated elder, and will take a family systems approach to determining what is needed to protect the elder, including looking at what services may be needed to support the care-giver and family. The Adult Services worker will likely do a thorough needs assessment, and can intervene to provide support, services or placement, even if the legal thresh-hold for a determination of abuse is not met.
It may also take numerous or repeated complaints before action is taken.
As Beer said above, the relative should be encouraged to report the situation. That is what they have the power and the moral obligation to do. They are not responsible for the outcome of the complaint.