Oh wealth of DiS knowledge, who knows about collection agencies and how to avoid them and how deep they dig for info?

Basically they're after money from the 4 month period before 3 of the 4 of us moved in, we told british gas that we weren't living there over the period when we got a bill and got a new one from the date that we supposedly moved in. Now we've had a letter from a debt collection agency wanting to find out more, and the one flatmate who was here at the time, and is therefore liable for paying up (her flatmates at the time are long gone), doesn't want to get screwed over.

So, should she lie and say she wasn't living here at the time in the hope they don't check the lease? She leaves in about 6 weeks time for the states and won't be back so its probably worth it.