Well, I think I agree with Matthew. Mark had wished for many years to be free of his wife but in control of her money; but each day and night was a new adventure - after all, she indulged him the membership in the expensive gambling and drinking club.
It must have taken a day or two to put together the "second" burglary, the swimming pool crime scene, and the ridiculous helicopter flyover!
I would think they were very close friends and Mark felt that he could really could help Hugh out by pinning it on the burglar due to his position. Probably realised pretty soon (when in bed that night) that he had a way to kill his wife and have a good alibi to not be a suspect.