She should seek a doctor's advice first. If she has never engaged in exercising before, the first trimester is NOT the time to start. As well as Redbone's post, the adominal area will expand and she will not have much core strength nor stamina since her lungs will not have the space to expand. Being pregnant requires ALOT of energy. She'll need 300 more caleries, without working out, (500 more if she breastfeeds) than before she got pregnant. She should seek other forms of exercise, types that involved endurance rather than strength training and perhaps you should get another partner.
On what do you base advising against strength training entirely?
Strength training is just as variable and adaptable as cardio, it has some excellent health benefits, and most of the special physical considerations (increased calorie needs, tendency to overheat faster, changes in weight and centre of gravity) are a factor in endurance exercise as well.
I found a couple good links on the topic (I've been reading a lot on diet & fitness lately, and a couple pages have info on training during pregnancy).
Weight Training During Pregnancy - from the website of a personal trainer and nutrition coach. She discusses common attitudes toward exercise during pregnancy and how they differ from scientific findings:
Early guidelines about training during pregnancy were largely based on speculation and expectations about middle-class women’s roles, rather than on clinical research and demonstrated results in a particular population. Luckily these guidelines have been updated and now reflect women’s experiences more realistically. The 1994 bulletin of the American College of Obstetricians and Gynecologists indicates that “[t]here are no data in humans to indicate that pregnant women should limit exercise intensity and lower target heart rates because of potential adverse effectsâ€...
...One of the important truisms about activity during pregnancy is that pregnancy is not the time to begin a strenuous exercise program. In fact, pregnancy is not the time to make any drastic physical changes.
However, women who are already accustomed to regular activity and exercise generally find pregnancy no disruption to their normal routine, and previously sedentary women can safely begin a moderate exercise program as late as the second trimester.
Also suggests some modifications and special considerations (specific exercises to avoid, how to listen to your body, etc). She mentions the joint laxity thing Redbone was talking about, and suggests shortening the sets while keeping the total volume the same (for example, substituting 3 sets of 10 with 6 sets of 5) if this is a problem. My understanding is this happens later in the pregnancy anyway, as the body prepares for delivery.
Cassandra Forsythe, a fitness and nutrition scientist, continued her weight training and interval cardio throughout her pregnancy. Here is one of her blog post on the topic:
Benefits of Exercise During Late Pregnancy
Perhaps you could ask your gym if any of its trainers are knowledgeable about the topic. Some trainers specialize in pre- and post-natal exercise.