Renters insurance company denied claim

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The property has a unfinished basement made of 100 year old concrete. In Vancouver WA you may know or suspect that we get a fair amount of rain. Well she left all the windows closed so the added moisture from the rain with no air flow caused mold/mildew to cover her stuff for the same reasons attics have soffits. Also, she had a lot of stuff crammed in there. All the way to the ceiling. 

 Previous tenants never had any issues but they had at least one window open and never had that much stuff in there. 

Stuff happens and that's why I require renters insurance which she has. But her company has rejected her claim and told her it should be on my insurance. She got a quote to clean her stuff: $2k. She can't use the furnace until it's resolved. 

I told her to figure out what wesel clause they are claiming and fight it. Tomorrow I am going to give her a notice to mitigate the issue within 10 days or I will at her expense as stipulated in the lease. I suspect she doesn't have the means to fix it and feel for her because I have been screwed by insurance companies too. 

That's all the information I have at this time. Any thoughts or recommendations are greatly appreciated.