His options are as follows:
1. Persuade the property owner to agree to mediation, which will cost them both £425 + VAT:
http://www.civilmediation.justice.gov.uk/
2. Instruct a 'no win, no fee' firm of solicitors to pursue the case. Start here to find a firm in the relevant area:
http://solicitors.lawsociety.org.uk/
(He might need to make a few phone calls to find out exactly what is offered by particular firms).
3. Find the £900 (based upon a claim for £20,000) required, possibly by borrowing it against a credit card if necessary, to use the online claims service:
https://www.gov.uk/make-money-claim-online
4. Find a couple of big guys with baseball bats and . . .
Options 1 to 3 are explained here:
https://www.gov.uk/make-court-claim-for-money/overview