RE: Ask for help. Filter a picklist field base upon a lookup field
There is not an easy solution, like there would be the other way around using pre-filter.
If you wanted to filter your lookup based on an optionset, you could use pre-filtering to filter the data in your optionset
In your case you would need to add an onChange event to the lookup and check for the value in the lookup (id or name).
From there you can do a switch statement to decide what to do with the picklist in each one of these scenarios.
I would recommend to see if you can replace the picklist with another lookup, or replace the first lookup with a picklist.
Hope this helps. If you have further details that can help us assist you please provide example of what you are trying to achieve, and why not use two lookups or option set as first field and lookup as second.