If I understood right, she is not pregnant at this time.
If the post-doc is a one-year appointment, I don't see a problem. Pregnancies generally last approximately 9 months. It often takes more than one cycle to conceive. If she wanted to be sure the birth wouldn't interrupt a 12-month appointment, she and her partner could always wait a couple of cycles before starting.
She may start to want to have an afternoon nap at some point; she'll probably want to bring some extra snacks to work; she would be smart to plan in such a way that she doesn't have to spend a lot of time on her feet in the lab in the last trimester. Some women find their concentration isn't completely what it was pre-pregnancy. She might want to build some flexibility into her project(s) in case she finds her productivity is a bit less than what she's used to.
Other than that, the only limitation I can think of would be if she works in a field involving radiation.
If the post-doc is a two-year appointment, then she may want to plan her work flow to allow her to ease back into work part-time after the baby comes, while still making progress with her research.
(How to combine work with care of a young baby is whole topic of its own which I won't get into here.)
Note, my answer would be different if she were about to jump into a tenure-track position, with a solid teaching load and having to build a lab program from scratch. But this position sounds nothing like that.
Whether or when to start a baby is a very personal decision. I see nothing objective here to stand in her way if she and her partner want to do this now.
(Note that in the U.S., employment discrimination against pregnant women is strictly prohibited. I can't imagine things would be substantially different in the UK. If in doubt, however, http://law.stackexchange.com can help.)