What is the purpose of her signing? Does she want her children to be bilingual (will be using exclusively ASL and will continue with signing on a continuous basis), or does she want to bridge the communication gap until her child's spoken language develops?
If she is using it as a communication skill, then, honestly there is not much she needs -- I wouldn't waste money on books or a class, etc.. All that is required is that she (and anyone who will be with her child on a regular basis) have a set of consistant signs that she uses. They need not be ASL signs -- in fact, using ASL can be detrimental to the process, since some require more dextarity than a child this age will have, thus defeating the purpose of decreasing frustration and facilitating incentive to communicate. I signed with all of my children. My first DD was the most into it. She developed a sign vocabulary of about 50 words by the time she was 15 mos. All I used was an old ASL signing dictionary to get ideas for signs. If the sign was too complicated, then I would modify or discard it and come up with something that my DD could use. I started with relevant signs -- ones for "more" "all done" "up" "please" "thank you" etc. As she developed an interest in signs, we added more -- objects she would point at, see in books, etc.
One thing that was slightly atypical with my first DD is that she held on to her signs longer than normal. When she started speaking, she didn't learn any of the words she had signs for until later (most children will start with the words they have signs for and then use speach and sign simultaneously for a period). I think for her, she felt she could already get her point across with sign, so she concentrated on learning "new" words, and only circled back to her signed words later.
Good luck!