Incidentally most people have around 1% methemoglobin when you check an arterial blood gas. If she had a 3% methemoglobin level, that's not too far from normal. I've seen 8% methemoglobin in someone who was asymptomatic.
That's a difficult question. She's not technically ill, provided she manages her flood levels. If her levels of methemoglobin become elevated, she can have severe breathing issues. Unmanaged, it can lead to serious health consequences. In a way it is analogous to diabetes, but far less serious.