My dad loves his P365. But the slide is really hard for him to rack. Any suggestions?

I have this gadget for my Kahr PM45 and it works.

Handiracker.com

Use an over-hand racking technique (as opposed to from behind the slide). Improved leverage, pushing into the shooting hand, instead of pulling with it.

Shoot the pistol more. Magazines improve with loading, and the recoil spring does get easier as the pistol is shot more.

This is the only recoil spring assembly for P365 that I have seen, that has optional spring weights.
https://zahal.org/product/sig-...-rod-by-dpm-systems/
Before purchasing, email/call them and ask if their lightest spring will decrease racking effort.

Hopefully, one of these days, the aftermarket will start supplying an un-captured RSA like they do for P320 Full Size.

If nada, then consider trading for the pistol that was designed for Easy Slide Racking...
https://www.smith-wesson.com/9ez

Simple...use the push-pull technique. Push the grip/gun forward with the strong hand while simultaneously pulling the slide to the rear. Bring the elbows/gun in relatively close to the body for more strength/better control.

Basic physics - you divide the force required 50/50. Keep the force vector parallel with both hands. In other words, don't pull/push up/down or sideways.

When I was teaching, I saw a number of small 85-100 pound women easily manipulate the slides on full-size pistols once taught this technique. Even GM 1911's.

I wouldn't screw with the RSA. Pistols with shorter/lighter slides rely more on the RS than the slide weight to properly function.