The heart rate thing is a crock of shit. Your heart rate will change depending on your current state of conditioning.
She needs to watch her TEMP while working out, not so much her heartrate. She should not get overheated, and she should remain able to TALK. Holding a conversation should be relatively easy. She should not be gasping for breath.
SHe needs to wear comfortable, breathable clothing. This is not the time for tight clothes while working out - her body need to be able to cool itself easily.
SHe needs to HYDRATE. SHe should have water with her at ALL TIMES, especially when working out.
She can continue doing whatever she was doing before - areobics, kickboxing, spinning, running, etc. It is fine to continue her routine with some modifications. Along with watching her temp, she needs to watch her balance. It is very easy to get off balance when PG - this is why most moves need to be modified. For example, in kickboxing, she would not kick as high nor move as fast. Also, on aerobics she needs to watch the "bouncing" - like with jumping jacks. Lower the intensity of this.
If she was not currently working out, at least not regularly, this is not the time to start a brand new gung-ho routine. SHe CAN do new activities, but keep it low. Things like prenatal yoga, elliptical, walking, stationary bike.
She can continue to do ab work through the 1st trimester (around 12 weeks). After this time, it is not safe to continue MOST ab work. After this point, her head should not go below her heart. So ab work on an incline is fine - I plan to continue using a stability ball throughout my whole PG.
Careful with stretching, she should not do extreme stretching. A hormone released during PG is relaxin, which relaxes ALL the tendons and ligaments in the body, esp around the hips. It is easy to pull something without much work. Stretching is a GOOD diea to continue though. Esp around the hips, legs and back. It can help with delivery and back pain.
PLEASE let me know if you have any questions. Since I am PG too, I have been doing a silly amount of research,
CONGRATS!
Oh, and pregnancy is 10 months, so if she is 4 weeks along, you have 9 more months!