He collapsed when they were leaving the stage in Philadelphia.
If you read Bob Seger's recent statements he says he was the one pushing Glenn to do the reunion, saying he would have fun doing it.
He was my oldest friend in music. I met him in 1966, and we recorded "Ramblin' Gamblin' Man" that year. We were just a couple of knuckleheads starting out – he was from Royal Oak [Michigan], and I was from Detroit. He was such a positive influence in my life. We'd always call each other for advice. I pushed him to do that Eagles reunion [in 1994]. He was the only one that didn't want to do it for years. I said to him, "I think you'd have fun."
https://www.rollingstone.com/music/f...-album-w514860
But the whole HFO thing, where he sang three songs, could perhaps suggest that he still wasn't entirely convinced about it until they actually started touring again. I don't know this. But to this day as a Frey fan it was one of my biggest disappointments to see them reunited & his role being so downplayed.