The willow will not know whether or not a liner is there. They do not sniff out water, but their roots will grow where there is moisture present. Where there is more moisture, the roots grow more. There is likely going to be a good deal of moisture in the soil between it and the pond. It is quite likely that it will be moister under the edge of the pond liner than away from it. If that is the case, more roots will grow just under the pond liner. As the roots grow there is likely to be damage to the liner.

I have a large weeping willow located about 15 feet from a pond lined with a butyl liner and haven't had a problem for 10+ years.

It seems to me the soil won't be moister around the pond unless there is a small leak in the liner or water is being splashed out from a fountain or waterfall.

Ted