A transwoman should definitely not be approaching a teenager in the lingerie department to offer help. It is completely inappropriate. The thing that stands out though, is the M&S response which puts responsibility for avoiding a repeat of this incident onto the mother and daughter, rather than taking action to ensure that a transwoman is not present in the lingerie department in future. There are umpteen other departments that this person could work in. Suggesting that the mother should phone ahead for a future visit makes her look like an 'awkward customer', someone who requires 'special treatment' and that they somehow have to pander to this. Never mind all the future teenage girls who might be approached in the lingerie department by a 6ft 2in transwoman. Because they'll be perfectly fine with it. Won't they?
Just to say though, there are far too many posters grabbing an invisible baton and running to the next person with it. There is nothing in the article to suggest that the transwoman wanted to measure the teenage girl for a bra or to touch her in any way. Apparently he offered help. I don't think he should have. And I agree with other posters that it is very unusual for any staff member in M&S to offer help unsolicited. And for this reason I imagine there's a very strong possibility that he got a kick out of playing a role and being all 'girls together', which is of course absolutely revolting. But, again, there is nothing to say that he wanted to ogle the girl in a naked state or to touch her.