Is she having the rapid heartbeat constantly? How bad is it? Is it getting worse? Better?
Switching from long-term use of 75mg/hr patch to a 50mg/hr patch could cause some (relatively "mild") withdrawal symptoms until the body adjusts, as it's used to getting more fentanyl than it is now. Rapid heart rate, chills and nightmares are are common with withdrawal.
Other common withdrawal symptoms are nausea, diarrhea, yawning a lot (with the eyes tearing up), runny nose, aches and pains, feeling unusually cold, sweating, trouble sleeping, goosebumps (at times when you wouldn't normally get them), and feeling depressed or anxious.
Rapid heartbeat is also a known side effect of fentanyl, but since she just lowered her dose I am inclined to agree with crashing that it is most likely a withdrawal symptom.
Are you wondering what she can do about it?