what a fascinating turn of events!
my first thought would be this.... "to what end?" what outcome are you trying to help her toward?
let's not beat around the bush, there's likely some element of sexuality involved here and so a misstep could be very sobering.
if your purpose is to help her understand that crossdressing isn't a life sentence to homosexuality.....do you know for a fact that's not the case with her son?
if it is to give her assurance that a crossdresser can lead a fairly average and ordinary life you might be just the ticket.
if it's to point her toward resources where she can be more accepting of her son, help him find his own way and the two of them maintain a healthy relationship......i'm sure you can do a great job of that.
the very best of luck to you and them!