It would be unfair to offer anything special to someone returning and not then also give it in some manner to those who never left.
Say he went through the user DB and reset every expired account where they've been gone at least 6 months and re-enable a trial for them. Add what ever flag you need so that those trials can actually last 90 days.
Would that be too insulting to current players? What do you get out of it? More targets to shoot at? A chance to reinvigorate the game you love? Is it really costing you anything?
I assume he has emails of past customers. If I want to reclaim my former id he has to identify me somehow. Those are former customers not random internet email leads. Send them an email and say, "As part of our 25 year anniversary celebration you have been granted a 90 comeback trial period. Comeback and remember what you loved so much.".
The trial is just sitting there waiting for them. That creates an urge to go collect the free thing I have coming. Once you get them back and rolling again, maybe some will stick.
Reasonably low effort. All the coding is in the non-graphic parts of the system HT can handle. We're not talking huge dangerous logic changes. Just some account parameter twiddling. A low cost attempt and changing the curve.
Would that not even be worth the try?