I had a similar problem (UK) it is actually very simply done, however, i cannot bothered to go upstairs and bull everything out the boiler cupboard to look at the wiring to colours are not accurate.
I am also not sure if the us/canada version is the same as the uk version.
Firstly, instead of using the existing wiring to complete a circuit use it to delivery power to nest thermostats
then disconnect all the old thermos from the boiler, but keep one pair to connect to the heatlink
due to it only having two wires on the uk heatlink there are number holes, 123 - 456 for heating and water
(from what i remember) he will need to connect the boiler to 1 and 3 and then bridge 3 and 2 with a one inch peice of insulated wiring and done
ive made some quick diagrams (dont judge)
** edit i forgot to label the last image, the 2 left wires are power i cant remeber if it runs on 12v or 120/230v, if 12 use the 12v power supply, if 120/230v jump off the boiler power simples.. and the other two are from one of the old thermostats
and for all you haters, this 100% as i did it myself nearly two years ago for water and heating and works flawlessy, (water tank) so yes it will do his water and heating if it needs to, it also has listeria protection for hot water tanks
****also he can connect as many thermostats as he likes to one heatlink and it all runs on wifi, he doesnt need a heatlink per thermostat