I posted this elsewhere in answer to a similar / same question. Particularily relevant if she used them in the cold – ime.
Sorry for being lazy:
It could be something else (but leaking caliper seals is a common problem ime with recent Shimano calipers).
To check – take the wheel out , pads out and jam somthing plastic between the pistons (a tyre lever works well). Then pull the lever till you get a real firm clamp. You will probably see fluid / bubbles from around one of the pistons.
Also check for any signs of oil on or near the calipers / mounts etc. Even a tiny amount is the start of real trouble.
How old are they – under warranty (12 months)?
I think for mine its something to do with using them in extreme cold – my winter bike suffers way worse.
If the piston seal is your problem (it could be something else!):
I’ve tried finding the seals on their own (the bit that seems to cause the leak) but cant be got. The good news is that you can buy a new SLX caliper for £25 from CRC which works fine (even with servowave levers).
Also a Deore caliper will work too a save a few bob. Installing and bleeding is easy, but you will loose about 1/2 inch of the hose (’cause you cant reuse the insert/olive)