Broken Glass on Bay Window

[Soleine Tholance]

Hello,

As I was inspecting a property for which the lease ends soon, my tenants showed me that the inside pane of a big bay window is broken - it isn't just cracked, a part of the glass fell. They admitted that it happened as one of them was leaning against it. They already got a (very cheap) quote to replace the glass. It never occurred to me that I would have to pay for the replacement, but the tenants are now asking if I could pay, because it might have been caused by, according to them, an act of god (temperature). It did happen in the winter as they had the heat on (and indeed the house was very warm inside). 

My first instinct was to have the tenants pay for the repairs. But a friend of mine says that I should bear the costs. A mentor of mine says the contrary : they broke it, they fix it. 

I did live in this house for two years and the window (double pane) has never shown any sign of stress - but no one ever leaned against the glass. 

All the glass companies that I called told me that if a big difference in temperature alone had damaged the window, it would have been only a crack on the outside pane. 

My lease agreement states that "Tenants shall keep all windows, glass, window coverings, doors, locks and hardware in good, clean order and repair". 

Considering all of these, who do you think should bear the costs? For a regular window, who should be responsible and in the event that parties disagree, who should bear the burden of proof ? Is it any different for a bay window - should it be expected to resist the pressure of people leaning against it ? 

Thank you,