Well if she has an appropriate visa that will allow her to return to the job at the end of the maternity leave (sounds like she might have if she is married to a UK national) then I would say yes they are obliged to grant the leave and hold the job open. If her visa status is in question, it might be a different matter, because the employer will argue why should they hold open her job if she isn't going to be able to return to it anyway? I suppose you can argue that by the end of her maternity leave she may well have her visa status sorted out.
Looking at it from another angle, I am guessing she is on a fixed term or temporary contract, in which case she is probably in the same position as anyone else on that kind of contract who is pregnant. (Can't say off the top of my head what exactly that would be, but should be pretty easy to find out - most likely they don't have to hold the job open).