Gene Hackman's wife was very serious about the star's health, putting him on a strict diet years ago ... so says his longtime friend and former business partner.

Doug Lanham, a close friend and former business partner, tells TMZ ... Betsy Arakawa made sure Gene was eating right -- demanding he stick to a strict diet.

In fact, Betsy took it so seriously, Doug says Gene was only able to sneak a cheat meal when they were out golfing ... this despite owning a bar and bistro for years.

Doug tells us he owned Jinja Bar & Bistro when he first met Gene ... who took an interest in the restaurant industry and became a co-owner in the enterprise eventually.

Hackman retired from the biz about seven years ago ... selling off his stake for a tidy profit, Doug says.

Despite leaving the restaurant industry, Doug says Gene would often come by for a meal -- because of the diet Betsy had him on he'd often order either the halibut or rice-paper salmon rather than a big steak.

Usually, when Gene came by he'd also pick up the tab for a few lucky customers too ... and, he even made all the art for the restaurant. So sounds like his post-acting years were still very busy.

Doug remembers Gene as a jokester who would play dumb little tricks on him ... like putting his finger to Doug's back and telling him to "reach for the stars."

Despite his love for the quiet lifestyle, Doug says Gene was always cool about sharing Hollywood stories with his friends.

As we told you ... Gene and his wife Betsy were found dead in their home Wednesday -- and, the warrant application shows the police feel their deaths have a suspicious element to them.

A caretaker of the neighborhood made the 911 call to police ... who arrived and pronounced the two dead at the scene. The investigation is ongoing.x

Gene was 95 ... Arakawa was 63.