Early termination of Lease by tenant

Need some advice from experienced landlords out there. Here's the situation.

I have a tenant who reported the discovery of bedbugs in their unit, on a bedspread and mattress. I responded immediately the next day with a pest control inspection by a certified exterminator and they found the presence of bedbugs. The soonest they could schedule the initial treatment was 6 days afterward. The tenant complied with the treatment procedure very well but the next day after the treatment sent a text that they could not live there any longer and they were moving out the next day. I believe they are blaming the bedbugs on a tenant 2 floors below them that I had evicted 3 months prior to them being discovered in the upstairs unit. The unit between these units had been vacant but was occupied for at lease 3 weeks prior to the discovery. There were some signs of bedbugs on a mattress left in the evicted apartment but I saw no signs of them while performing renovations to the unit. The end of the lease term is still 3 months away. The tenant did not give proper notice in accordance with the lease and what are my obligations to agreeing with the early termination? I believe I have the right to hold them to the lease terms but also want to do the right thing. Any other experiences similar to this. Any advice would be appreciated. Thanks