If your existing unit is already equipped with an HWG (hot water generator) you shouldn't have much of an issue using it for pool heating.
However, I wouldn't expect a very high btuh output. They are designed to provide heat to a domestic hot water tank, not the size of a swimming pool. They are also designed with a triple layer coax coil, incorporating an atmospheric vent between the refrigerant and water lines, this is to eliminate the risk of potable water contamination with refrigerant.

There are also aftermarket HWGs that can be installed on any AC or heat pump unit. I wouldn't suggest installing one unless you're knowledgable on refrigeration.

My guess is, the old contractor was suggesting to have the ground water loop circuit rerouted to your pool water? I'm a bit concerned about this approach, with the potential for refrigerant contamination. It is a slight risk, but a risk none the less.

A great concern would be routing the ground loop circuit to a hot tub. A large refrigerant leak in a small environment could be extremely hazardous.