The landlord has a duty of care toward anyone on his premises and should not have allowed this incident to get so far out of hand. What would have happened if you had been there and been injured? It is quite normal for someone in this position to try and offload the blame but a good solicitor should concentrate his mind on his responsibilities.
Explain to your friend exactly what happened and what steps you are taking to get compensated. Check with local music shops to see if you can find a similar instrument and, if so, check whether your friend wants that or the option of the cash to buy something himself. If this is not possible in the timescale do you know of anyone who will loan a guitar for his gig, so long as the instrument is kept under strict guard?
He probably won't be too happy but if he can see that you are doing all you can to make amends, even though this was not your fault, that will probably sugar the pill.