Unfortunately, Excel does not have an option to skip negative or zero values in a logarithmic scale chart. However, there are a few workarounds that your client can try:

1. Replace zero values with a small positive value: Your client can replace the zero values in the data with a small positive value, such as 0.1 or 0.01. This will allow the data to be plotted on a logarithmic scale without affecting the overall shape of the chart.
   
2. Use a secondary axis: Your client can create a secondary axis for the data with zero values. This will allow the data to be plotted on a linear scale while the rest of the data is plotted on a logarithmic scale. To do this, your client can follow these steps:
   
   1. Select the data series with zero values.
   2. Right-click and select "Format Data Series."
   3. In the "Series Options" tab, select "Secondary Axis."
   4. Adjust the axis scales as needed.
3. Use a different chart type: Your client can try using a different chart type, such as a linear scale chart or a stacked bar chart. This may be a better option if the zero values are important to the overall analysis.

I hope these suggestions help your client find a solution that works for their needs. Let me know if you have any other questions or if there's anything else I can help with.