OP - I agree stick to hospital wards, otherwise you could get bogged down in theoretical stuff and you'll run out of time.
Ask why we segregate on the basis of sex? (safety - men present a greater risk to women then women do to men. Also, dignity, comfort, women have biological functions that men do not).
Point out that (i) Majority of transwomen retain male genitalia (ii) Research indicates that transwomen retain a male pattern of criminality. (iii) 1 in 3 women have been victims of sexual assault or violence (mention 'Me Too' etc.)
State that whilst we're not saying that all transwomen will harm women, ask her why, given the research available, women should continue to feel as safe with transwomen in women's spaces . . .
Point out that in many psychiatric wards now transwomen are being admitted without evidence of a GRC, any medical treatment or any gender dysphoria. Men who dress as women on a part time basis now self-define as 'trans' (thanks to Stonewall's trans umbrella) and are admitted to female wards.
More than 2/3 of women on psychiatric wards have been victims of male violence/abuse/rape. Bad enough, but these vulnerable women - many with psychotic illnesses - are being told that transwomen are not men, but women.
These women are told they do not have the right to feel uncomfortable with these 'woman'. They are labelled 'transphobic' and threatened with removal from the ward.
Ask Stella whether she would feel comfortable and safe, at the most vulnerable and frightening time in her life, sharing a ward with someone who looked like a man, spoke like man, dressed like man and had a penis. Ask Stella to ask her female friends the same question.
I don't know Stella's sexuality, but ask whether she and her female friends would have a sexual relationship with a transman/woman.
She will fudge it, but try to get across the point that if no/not sure then she can't truly believe than transwomen are women. So why on earth should vulnerable women in hospital wards?
Good luck!