The absolute easiest way is to put two new holes and fittings into the spa and plumb it in as a COMPLETELY independent system. Anything else, you'll go bald pulling your hair out...bald from head to toe.
I"m assuming they're heating water directly...rather than a PV system? if so, how much storage capacity do they have ?( FYI, unless they have MASSIVE storage capacity, never in a million years will they ever reach payback)?