I charge according to what it costs me - so if there is some way that doing the two jobs can save me money, then (and only then) do I pass the savings along to the customer.
As to 'should he take a free..." it sounds far from free. Add up all the costs and then compare to buying a new spa. Keep in mind that if the cover is old you will cost yourself energy - and may have trouble keeping the tub clean and hot. However, if you buy a new spa, you get a new cover, a warranty, a new control board (duh :-) and the delivery is generally included at no extra cost. The electrical and ground prep would most likely be the same, although that is one area I will give things away: if it helps me to sell a tub I will do electrical and slab work near my cost which is lower than most contractors could do it.
So - if they can put a new cover on it, move it and then repair it for a reasonable cost, they will have 'gotten into hot water' for less than the price of a new spa. But if you add up all the costs, and then if the thing turns out to be an energy hog or for some other reason is more costly to own than a new tub, they may not enjoy it long term. Some people - at that point - would still have considered it a good investment because all the electrical work and ground prep is there for a new tub, and they may get some trade-in because the old tub is now working.
Just some thoughts - HTH