What's the output impedance of the AD8231? I want to use it to drive a ADC (ADS7952) which recommends <50 ohm source impedance
The output impedance of AD8231 is very low, though I can't say what is the exact value, the recommendation of <50 ohm source impedance will be satisfied by the AD8231.
In addition to that, you need to consider if the amplifier can deal with the capacitive loading imposed by the RC filter between the amplifier and the ADC. AD8231 is very good in handling large capacitive loads, kindly refer to the TPC section of the datasheet and look for the responses of the part at different capacitive loads.