Can I block customer's gutter?

I've a fsg job booked in on Monday and the customer has asked me to divide her gutter from her neighbour's by blocking it off.

She has new uPVC while her neighbours are old iron ones and she has leaks where they've been joined.

It's not the first time I've been asked to do this for customers so I have two questions:

Can I do this?

Should I do this?

Thoughts/experiences would be much appreciated!!

Only if the other side has there own outlet.. You may need to reset the fall..

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Only if the other side has there own outlet.. You may need to reset the fall..

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Good point.

Can't you seal the leek instead?

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CT1 sealant?You do not need the potential liability for next door./emoticons/wink.png

The liability is the biggest issue I'm concerned with - as @Alec pointed put, the fall of the gutters may be affected and I have no experience in setting (?) them.

@Hunty the customer has tried sealing them unsuccessfully. Sealant may be cheap stuff though?