I thought the same. Unless it's an intimate affair (probably wrong choice of words) there should be more than enough room among the hundreds of attendees for them to avoid each other, which as adults should be possible. TM was being a bit of a drama queen with her demands IMHO. Or perhaps she'd heard so much about Allison's supposed histrionics that she believed she'd have a melt down if they were to catch a glimpse of each other.
I think from the previous encounter at the gym, TM couldn't handle the guilt and probably her own envy of the legitimate relationship between Allison & G. I think she might have, thought if she could get G to mention to Allison, Allison would know G is still in touch with TM and it would end things befween A & G once and for all then TM & G wouldn't need to sneak around and TM can be #1.