Bed bugs - unfurnished flat - who is responsible?

Maintenance

My tenant and her two children moved into our unfurnished flat three months ago.
Today she called to say her brother, who was staying over, got bitten by a bed bug. She wants the flat to be treated.
My tenant or her children have not been affected during her time there.
The flat was let unfurnished and has no carpet.
This is her first complaint to us, although we have raised concerns with her about damage.
We are willing to help, but are unsure what we are required to do bearing in mind the flat is unfurnished.
And while we want to nip this in the bud, we are sceptical at this stage that it is an infestation.
We've had the flat for eight years and no previous complaints about bed bugs etc.
Any advice on this would be very welcome.