How's that relevant? If she were there to negotiate, he's done one instead of representing his party.
And if my nana had balls, she'd be my grandad. It's the only thing that's relevant to me. The actual progress of things, rather than appearances.
Does he look childish for this, yes in my eyes. But it's entirely inconsequential and was always going to be. May just wants to push on with her deal regardless of what other leaders say.