I forgot to mention that the heart appears to be normal on the ultrasounds.
A fast heart beat (tachycardia) can be harmful if the sustained heart rate eventually results in the heart "tiring out", or if the heart rate becomes so rapid that the heart cannot "fill" effectively and therefore blood does not circulate the way it should.
The perinatologist and pediatric cardiologist will monitor the baby carefully for signs of these effects, and may give your daughter-in-law medications that will cross the placenta to treat the baby. Digoxin is one of the more common drugs given for this purpose.
After the baby is born, further evaluation will be performed by the cardiologists, possibly including monitoring of the heart's electrical rhythm by EKG, repeat heart ultrasounds (echocardiograms), and in some cases cardiac catheterization.
I'm glad to hear that your daughter-in-law already has the right specialists helping her to optimize the management of the baby.