In depression patients, if Ritalin is prescribed for ADHD but not taken properly can it cause depression?

My girlfriend has depression and ADHD. Her doctor prescribed her depression meds which she doesn't take. I know she should but she has been doing great without. Recently though she has been prescribed ritalin for ADHD. Thing is she often forgets to take it but she sometimes does. She has been very depressed lately. To an alarming point. So could the ritalin be causing extra depression and could the depression meds and proper use of Ritalin fix

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