Whilst it is clearly a ball-ache for you it might be worth telling her to get her boiler checked as clearly something is letting out potentially deadly CO in her house.
I'm going to have a chat with her this evening to tell her we heard her alarm go off last night and that we haven't had a fire so she should get someone out to check her boiler ASAP, because I would put money on it that it's her boiler causing the problem, not our fire/chimney