What Causes Blisters O The Heels?

Hi, My daughter is wheelchair-bound with fragmented feeling in lower extremities. SHe developed a blister on the back of her heel that I thought was caused from the tennis shoes she was wearing. The blister was about the size of a nickel to quarter and slightly raised this morning. I covered it with gauze and over the counter antibiotic cream this morning. I had her wear only soft loose fitting socks today. She has been in her wheelchair all day with no contact at the site. However, I just removed her sock and gauze and the blister is the size of a half-dollar at least and significantly more raised. Is this common for friction/pressure blisters? We usually just have pressure marks when a shoe fits incorrectly. This has never happened before. Do I puncture the blister? Elevate? Apply any sort of cream or ice? I appreciate any advice. I try not to take her to ER unless absolutely necessary as she tends to catch something else in the ER. However, if this is something that should be seen before the morning I will. If it can wait, I will take her to the doctor in the morning. Thank you for taking the time to read my long message.