Yeah, it can happen. Folks can burst through the symptoms for the short-term for various reasons. The important thing is to look for long-term patterns.
My XAH is bipolar. Manic, he was the funniest and most charming guy in the room. So tall and good-looking, really cool, HILARIOUS, great sex, just a riot. But mania and depression are like getting a check for $1200 after the best meal of your life. At first, with mania, you get the dump of all the good chemicals and feel like a million bucks, you can do anything, be anybody, then your body tries to cope with a lack of the feel-good chemicals while it tries to make it all up, and struggles and struggles and struggles to do so. My XAH was very irritable and anxious, always manipulative, pessimistic, and combative, regardless of where he was in his cycling. The mania made it seem less dangerous somehow.
Mental illness or no mental illness, people are still accountable for how they treat people. If he's manipulative and conniving with her, eventually she's going to have to make a call about whether she wants that in her life, whether he's mentally ill, alcoholic, medicated, not medicated, or whatever. Controlling is controlling, whether he's doing it with guilt trips or his fists.