Semi detached roof structure problem!

[Icantstopeatinglol]

Hoping for some advice on what to do in this situation.
Our neighbour many years back was growing ‘plants’ aka cannabis in his loft. He (for some ridiculous reason) took out all the internal roof truss’s (I think that’s what they’re called) out! The police were called etc but he sold the property to a friend and we thought it had been sorted. However, the people that are renting the property have just told us that it’s still exactly the same!! We’re now worried about how that could affect our property. He’s trying to sell it but I can’t see how he can in that state but then that’s not my problem! What can I do to get him to fix this? The police would have been aware at the time but not sure what they would have said about it?
Any advice is appreciated.

Is their roof (or yours, come to that) showing any sign of sagging? If not I honestly wouldn't worry about it. If his roof collapses it's their problem and I can't really see how it would affect yours, unless of course they didn't repair it and left it open to the elements, which means you might get damp problems. Your roofs are separate entities surely, and your trusses are all in place? Are you thinking of selling in the future? If it were me, I would be very careful about getting involved in this because you may end up having to declare it when you sell.(if, for example, you had made a complaint to the council about a perceived unsafe roof next door) .
If your neighbours landlord sells, it would be picked up on even a minor survey, you would hope, and probably put right.

It's not a police matter but the building control would take an interest. If the work was not done to building regs then the council can take enforcement action if it's unsafe.
If it does collapse for some reason, make sure your building insurance is up to date and they should take legal action on your behalf, and pay for the damage in the mean time.
It's up to the buyer to get their survey done and decide whether to take on that liability or not.