Hey you're right they don't specify who's ally it is. Great call. .I was curious about how they get a message to the Ghostbusters too. The only thing I can think is Casey and April witness the Collectors capture the Turtles and they're the ones that enlist the Ghostbusters help. .Good call.
I have been trying to catch up with the events of the Turtles comics in preparation for this crossover so I have no idea what Ally they may be referring too. By hey have made a lot of friends in these 70 plus issues. I am happy to find out that the events of the first crossover have been referenced a few times. In the Turtles comic The Immortals were having a banquet that they have every 500 years. They mention the absence of Chi Yu. I can't wait for that Ghost Trap with him in it to possibly make it's way back to the Turtles universe.I'm glad someone's doing that. heh. Please feel free to list the issues the first crossover is referenced.
As for the long lost ally - I couldn't but wonder if something happens to Jenny in 101 #6. Or maybe Ray's Spirit Guide turns on them. He certainly would like what's happened with the multiverse after the warnings he gave as far back as Volume 1 of the ongoing series.
Of the issues that I have managed to get my hands on the first reference to the first crossover was in TMNT # 41 Donatello is working on the Transdimensional portal and he tells Harold something like. "Make sure the portal is calebrated properly this time Harold, don't forget what happened with those Ghost Guys."
The other reference was in issue TMNT # 72. That is when the immortals mention Chi Yu a few times. First they mention his abscense for the past 500 years. The Rat King mentions that Kensu was the last one to see him before he disappeared. The immortals do a role call and Chi Yu is mentioned again. I am paraphrasing again here and I am sorry, but they describe Chi YU as a fearce warrior and unstoppable. The Rat King makes a snide comment that since Chi Yu has been gone for 500 years the "unstoppable" part may be debatable. . Those are the ones that I have personally found, but I am sure there could be many more references to the event because there is not just the 72 issues but the Spin Off Titles as well. Here is a chronology chart, but I don't think I am going to be able to catch up completely to see if the event was referenced in those titles.