I am a runner/triathlete mom. I wanted the "Running for Two" shirt but wouldn't buy it for myself.
Does she have a running stroller? If so, or if she has registered for one, accessories might be nice, namely a rain cover and uh, what's it called, the thing that holds your water bottle/phone. There is also a universal phone holder that I got on Amazon.
Books are another idea. There are a few good titles on running/fitness during pregnancy.