Good core workout for someone just starting?

edibor
My SO has hip/back problems and has been told to strengthen his core to help support his frame. We're looking for a core strengthening program that is not too aggressive (he's in pain) that is not too time consuming so we can do it every day or every other day. I personally think crunches are really dumb, because they strain your back and there are a number of ways to do them wrong and reap no benefits. But other exercises (like the one where you lie on your back and bring your face toward your opposite knee) should be fine.

Any suggestions for either existing regimens/classes or just a few exercises that would be good for a slightly overweight guy in pain to strengthen his core? Can we do just the core days of a P-90X-ish workout program, for example? Thanks!
Good core workout for someone just starting?
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