Rats come in for all sorts of reasons, normally food brings them in in the first place, but when we had our rat problem - under nextdoor's decking! :roll: they simply ate all the frogs, worms and slugs/snails in our garden. We had removed all bird food by then but it made no difference. Rats will eat anything.
Your friend needs to do several things:
Fill in the holes under the shed everytime they appear - rats can shift huge amounts of soil - they did under our shed.
Remove any food that is accessible to the rats. So the shed needs to be made rat proof - all holes mended/sealed off and make sure they can not get in under the door etc. Buns need ventilation though so she will have to figure something out.
Remove any bags of compost, waste, junk, anything around the shed or garden that the rats could move to. This means cutting back bushes and scrub and basically tidying your garden up. They will move elsewhere... but can and will still visit so ALL source of food need to be kept out of reach. Put rabbit food into sealable storage containers with lids and only feed rabbit food in the shed itself. If the buns are outside at times - as already said only give them hay outside - hay/grass throughout the day is ideal for buns, they do not need food outside. Rats do NOT eat hay!
Remove any bird feeders/food for the time being.
Bait boxes and bait can be bought from places like B&Q or Scats countrystores etc. You must wear gloves when handling bait and wash hands afterwards. Fill the bait box as directed on instructions somewhere well away from rabbits and over something that stops it dropping on grass/ground, put a sheet down or do it over a bin or something. Seal the box and put it somewhere the rabbits can not get to it - round the back of the shed, behind the fence or if you leave the holes under the shed open you can leave it facing into the hole so they go straight in. It takes a while to work but it is very effective. You have to use sufficient bait to prevent resistance to the bait.
We also used traps - personally I feel they are much quicker and more humane than bait - but we had a very strong rat here that broke both heavy duty traps and got away:shock:. They work well on the young however.
I hate this whole thing, I couldn't do it, my OH had to deal with it all as killing anything upsets me. The rats were in our neighbours garden and under their decking and all the gardens were being overrun. One neighbour pointed out it must be our rabbits bringing them in - until I told her that I don't feed rabbit food at all - that shut her up! :lol: It was nothing to do with our rabbits, they were coming in to everyone's garden for bird food amongst other things, and with such competition from so many rats, they ate all the insects/worms/frogs etc instead. We even had a fish go missing from our little pond.
Very sad really
But unfortunately has to be done.