Allegations

Making a police complaint against them for trespass and nuisance, issuing a legal notice denying all that allegations are the only remedies.

file a police complaint. alternatively, you can ask them for proof or the context in which they are making the claim and try to find out if there is any truth in the allegations. this way you may want to relieve your late father of such burden. 

Yes, a police complaint complaint could be made.

As per your query, I would like to tell you that you're friend should first file or lodge a police complaint against his uncle's intolerable behaviour towards his family, such that a warning sign and pre-requisite process is complete, however, if the police complaint does not make an impact then he should proceed to file a civil matter against his uncle's under section 499 and 286 of the Indian Penal Code for defamation as well as for creating nuisance respectively.

⭕ Section 499 of IPC, 1860 deals with the offence of Defamation. It states that -
"Whoever, by words either spoken or intended to be read, or by signs or by visible representations, makes or publishes any imputation concerning any person intending to harm, or knowing or having reason to believe that such imputation will harm, the reputation of such person, is said, except in the cases hereinafter expected, to defame that person."

⭕ Section 286 of the IPC, 1860 deals with Negligent conduct with respect to explosive substance. It states that -
"Whoever does, with any explosive substance, any act so rashly or negligently as to endanger human life, or to be likely to cause hurt or injury to any other person, or knowingly or negligently omits to take such order with any explosive substance in his possession as is sufficient to guard against any probable danger to human life from that substance, shall be punished with imprisonment of either descripttion for a term which may extend to six months, or with fine which may extend to one thousand rupees, or with both".

Hope It Helps

Regards
Kevin M. Paul