Heart palpatations and irregular heart beats are a serious side effect of the medication. Since your friend has been taking percocet for years, he/she is addicted to the medication. When the medication starts to wear off, he/she is experiencing a withdrawal which can cause heart palpatations and irregular heart beats due to anxiety.
Here's a list of side effects and severity:
http://www.webmd.com/drugs/drug-7277-Percocet+Oral.aspx?drugid=7277&drugname=Percocet+Oral&pagenumber=6
Also, Percocet has a severe potential hazard to your liver. Here's information from http://drugs.com website:
Acetaminophen (Including Percocet) is primarily metabolized in the liver to inactive forms. However, small quantities are converted by minor pathways to metabolites that can cause hepatotoxicity or methemoglobinemia. Patients with hepatic impairment may be at increased risk of toxicity due to increased minor metabolic pathway activity. Likewise, chronic or overuse of acetaminophen can saturate the primary hepatic enzymes and lead to increased metabolism by minor pathways. Severe liver injury, including cases of acute liver failure resulting in liver transplant and death, has been reported in patients using acetaminophen. Therapy with acetaminophen should be administered cautiously in patients with hepatic insufficiency. Clinical monitoring of hepatic function is recommended. Instruct patients to avoid drinking alcohol while taking acetaminophen-containing medications. Patients should be warned not to exceed the maximum recommended total daily dosage of acetaminophen (4 g/day in adults and children 12 years of age or older), and to read all prescription and over-the-counter medication labels to ensure they are not taking multiple acetaminophen-containing products, or check with a healthcare professional if they are unsure.
*It's very bad for your liver to take Percocet on a regular basis for years. I hope your friend hasn't been drinking while taking Percocet as it is especially toxic to the liver.
I recently found out I have liver disease and I am only 31 years old. I took percocet for a few years, along with anti-depressants, and drank heavy in my 20s which caused liver damage with fatty deposits and enlargement of my liver with hemangioma. I was told it is irreversible. I'm now on an extremely strict diet and have to be careful the rest of my life so it doesn't develop into cirrosis.
Please tell your friend to seek help to get off this medication. Absolutely do not drink with it. Besides harming your liver, you can also overdose (especially when taken other ways besides orally). Your friend should also have his/her liver enzymes and cholesterol/tryglycerides checked when going for a check-up and blood work.
Percocet is composed of an opiate and Tylenol. Long-term use of Tylenol can cause liver, kidney and heart problems. Your friend had best see a doctor ASAP.