Audax rides

it could still be counted as a 1000k DIY

Yes, because it's not a race.
You should notify him on the entry form that it is an organised event.
Rules are here:
https://www.audax.uk/about-audax/event-types/do-it-yourself-diy-events/diy-faqs/

Other cycle events may be concurrently ridden as DIY events providing the normal AUK requirements for DIY events are met. Riders should submit a DIY Perm application to the DIY Organiser and subsequently submit ‘Proofs of Passage’ for validation as normal. Include details about the ‘other’ event in the additional information area of the DIY application form. Where the organising body ‘validates’ the riders participation in the event, such validation might, with the prior agreement of the AUK DIY organiser, be used as ‘Proof of Passage’ for the AUK brevet. Factors which need to be considered in assessing whether the external event is eligible for inclusion in an AUK brevet include:
Distance - the ride must conform to AUK regulations regarding controls, distance and the collection of appropriate ‘Proof of Passage’.
Support - this would follow the usual practice for audax events in each country. In the UK, personal support is not permitted outside of designated controls and riders travel to support, not support to the rider.
Competition - the event must be non-competitive, as required by AUK regulations and insurance.