I'm happy to write a letter she can bring with her, but I want to know if there's anything else she and my dad should bring with them? Bank statements? They've got e-tickets, will just their itinerary be sufficient?
Yes, young women visiting boyfriends or having entered the UK frequently as a visitor or on other visas coming back look like they may want to overstay their visas. Assuming your Mother doesn't have a diary filled with details about how much some young Brit means to her and how she just can't stand to be apart from him, etc, or has been staying up all hours chatting on the Internet with some guy she's never met in Bristol, she should be fine.
A printed itinerary should be more then enough in their case, but if you are worried, sending her a letter of sponsorship (saying that you and your husband are inviting her for X period of time and will ensure she is provided with accomodation during this period) then she can keep that as backup.
What is likely to happen is:
IO: "What is the purpose of your visit?"
Mom: "Visiting my daughter who is pregnant with our first grandchild."
IO: "Oh, isn't that sweet, how long you plan to stay?"
Mom: "A month"
IO: "Oh, you going to get any sightseeing in... blah blah blah" *STAMP* *STAMP* "Thank you and enjoy your visit."