You/he need to think through how the system will be operated. Is the forced hot water system going to be kept on all summer to heat the indirect heater? That tends to waste energy.
If the indirect heater is after the gas heater, then there is nothing to keep it hot during extended periods of no usage like a long weekend away.
The indirect heater could be first and the forced heating system could be used when there is extra demand.
You could install a small circulator for the hot water between the gas water heater and the indirect heater. That would provide storage while firing either unit. Firing only one unit will save some energy.
Success starts with the planning. If the owner doesn't want to be bothered with operating details he is never going to like it.