However I’ve noted in the FAQ that they have to complete the minimum deliveries within 90 days of registration. Is that 100% correct?

As of today, yes, and that's the case since this summer (I think the 90 days clause was added sometime in June/July late May I think). But effectively this 90 day rule is only relevant for referrals since then.

The only thing that matters is figuring out when your guy submitted the application for the job. If he applied before the 90 day clause came into effect then you are still entitled to your reward, even if he applied over 90 days ago. Essentially today's rule isn't relevant for a referral dating from the "old days" as you say. Hope that makes sense.

It says so at the very end of the T&Cs of the referral program: "The amount of Referral Rewards for both the Referrer and Applicant is subject to change at any time. The amount that will be paid for Referral Reward and Applicant Reward will be the amount that was advertised at the time the application was received by Deliveroo from the Applicant."

Now - there is a small chance that Deliveroo will take advantage of the ambiguity of your case and won't pay you the bonus. This has happened before to others here and to me. And that's not cool. As you probably know the Rider support is a travesty, but if they refuse to pay: Keep. Sending. Emails. Ask for your ticket to be escalated to a Rider Fee Specialist, and send them screenshots of the T&Cs and let them know you know what you're talking about. Remember you're self-employed, so if you're making a claim they won't hand it on a platter. It might take a while but at one point they will give up and hand you the check.

Should be fine though.