A couple of things worth trying; it might be that her "let down" reflex isn't letting her milk go as freely with the pump as with baby which is really common so she could try hand expressing a few drops before firing up the pump, or her boobs might be more giving if she's got baby close to her - perhaps holding him or even pumping from one boob while she's feeding directly from the other (a bit fiddly!)
I'm sure you've all checked it over but it might be worth double checking that the pump is put together properly and definitely not faulty. I tried an electric pump and also didn't get on with it - I found I had much more control over the sort of suction with a manual one and got on really well with it (I only really expressed when we were going out, or when I was after a lie-in so daddy could feed him, if your daughter-in-law is planning on expressing lots if feeds I think a good electric is probably faster).
If none of the above help, the nct breast feeding helpline is brilliant - I phoned them twice when I was feeding and they were really lovely and didn't make me feel daft which is what I was worried about - I'm sure they'd have some good tips.
Wish her luck and give her a big well done for sticking at it!