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Just dump some weedkiller on her plot and tell her what you have done. If she doesn't like chemicals then she needs to get her plot cleared as you haven't got time to be weeding her plot also.
For God's sake, it's couch grass, it's not ebola. I'd hate to have a plot on your sites. If yours is perfectly worked, good for you, you've a lot more time on your hands than I have, which does not disqualify me from having an allotment. If it's rough it's because she's clearly not got a lot of time. If it's so bad it's seeding, have a word and get her to strim it. She may not even know that her weeds are a problem for other people - you don't until your own plot's clear. Otherwise, dig a trench.