Advice needed please

I help run a yearly convention. A year and a half ago or so, one of our attendees transitioned MtF. As far as I know, she has had no troubles, other than some wrong name usage (she has been coming to the convention for 10+ years). The wrong name usage dropped from last year to this year. Even my very conservative friends have treated her well, doing their best to use her correct name.

The problem is that she will wear his old clothes, and to be honest that is very confusing. The shapeless shirts and such leave her looking pretty much looking like he did, especially from behind. What I would like to know is would it be appropriate to talk to her about this, and how would I bring it up. I won't see her for another year, so I have plenty of time to figure this out.

So should I, next year approach her or not?