Bed sores on butt.

Can be deadly, especially if it's a Kennedy Ulcer which usually appears on or near ones tailbone.
Nothing to mess around with, so please get medical help for this person ASAP.

Pressure sores can get nasty very fast. If the person is bedridden, they have to be turned every two hours. These sores can go stage four really fast where they are eating into the bone and a bone infection. Sepsis is a real concern so they need to get on this fast before it gets deeper. The person might night IV antibiotics as well as wound care. Wound care will need to bandage and change it properly as well as keep the right amount of pressure on it. I agree, medical attention asap.

Pressure sores can be VERY dangerous.
To help prevent them reposition the person at least every 2 hours.
Indecently that is about how often you should check a brief (aka "diaper")
If the person is in bed an alternating pressure mattress is advised. If that is not possible moving a person from side to side or even pulling the sheet up or down to move them a bit.
Also try to remove as many wrinkles for clothing or bedding.

A pressure sore needs to be treated by someone that knows what they are doing. It is not a simple "ointment and bandage" situation.

the persons doctor can refer a Wound Nurse or if the person is on Hospice contact Hospice right away and let them know a pressure sore is forming.

There is a pressure sore called a "Kennedy Ulcer" that can appear typically lower back, upper buttock area, shape of a butterfly. If this appears it will be 24 to 48 hours before death.

I agree with giving her immediate attention. Where is s/he located? If in a facility then stand at the nurses station and insist on getting her seen asap.

If at home and she is mobile, to the ER. If not mobile, call 911.

OH OH! Off to doctor at once. Decubiti can be deadly. They can cause sepsis and death.
This requires special wound care measurement, treatment and followup. Not a do it yourself project. Call the doctor today.