First of all get a 2nd opinion in the repairs ... ring the manufacturers direct as they have specialised contractors and i've had cases where it is easily diasgnosed as something different just because that certain plumber hasn't worked on that make before (hope that makes sense?)
After that, depending on what he says, speak to the tenants direct and be truthfull and say that you would like a contribution if not all the money for the repair .... ask the plumber in question to do a report of whats happened and why to back your story up. If it is mega bucks to fix it then you can always get the small claims court involved if they dont play ball. Just a mention of this may make them pay.
If the plumber does do the report then you have evidence that you can use the deposit held to help with the cost of this, if the tenants do move out or are given notice. Ideally you dont want an empty property so they being calm with them and talk normally, face to face if possible.
These things do happen but not to this extent .... the agent is in the wrong and personally i would leave them at the earliest opportunity.
Keep us informed.