That's what I was hoping. Mind you, she's more likely to be on a slightly later train from St Albans given that she usually runs late - lucky that the cross London times always err on the generous side!
Aim for an earlier train then, so that if she misses that she'll be on the suggested train.
As MikeWh says, it's risky leaving less than the (admittedly generous) transfer time.
Last time I was at St Albans, a train was leaving southbound as I arrived but I didn't worry, thinking that trains are frequent enough. After over an hours's wait, with the previous four trains all cancelled, I was regretting not checking live departures before heading out... If that had been my 'booked' train leaving the station, it would have been a very expensive mistake...