actually someone ask me that question once over youtube. when I read the "official" subtitles it is possible that at the last moment Dante was going to give back Gluttony's memories via alchemy however once she hit Gluttony with the alchemy she ultimately fell to her demise.
In this case Gluttony fell too and probably couldn't climb back up so he ate Dante's remains and what was left of her PS which probably resulted in the mutation. that's the only explanation if you believe Gluttony could has reconized Wrath.
though I tend to agree moreso with Merci Alchemist94's theory, in which is what I responded to the fellow on youtube. chances are it was simple irony of him seeing Wrath first.