She isnt ill, the inability to remain faithful is not an illness, its a character defect.
Actually, yes and no. It depends. Sadly, it does. Medication or lack of medication can cause a person to go "crazy" or have thoughts they would not normally have.
The other factor is that every human has the capacity and natural mental ability to have an affair. Even here we talk about how getting close to co-workers can accidently start that path not the same as the guy (or woman) who is on the hunt to find someone to cheat with.
Early days of post-D day, I went through 3 different kinds of medication before a doctor got me on something that was stabilizing and non-addictive. I did Xanax for a while but had given someone else control of the pills for fear of becoming addicted. But Xanax can usually be bad for depression. Did Prozac for 2 days, stopped when sudden abnormal thoughts of suicide started.
Drinking can severely effect medication and of course cause death. 6+ months before the affair, my wife's doc. gave her a different medication - which over the course of a few days didn't know how bad things were getting until it blew up. Police were called - she ran away, she had accidently caused self-harm by falling over a railing. Stop that medication cold. It was talking to the cops who said "has she started taking new meds?" - light-bulb.
Now, the thing to keep in mind is that people who are bi-polar seem to be higher with cheating... and can become addicted to the AP much harder than a non-bipolar person.
None of this should give the cheater a "get out of jail free" card, of course. That is up to the BS / WS to decide and what damage has been done and what is tools are being used for recovery.
Those without mental illness, have less excuse or reason for their actions.