Why do you believe your client can exclude $4,500?  For MFS, the maximum exclusion is only $2,500.

PTO is a dummy when it comes to DCB for MFS couples.  It only understands dependent care credit but not DCB exclusion.  This was brought up numerous times to the developers but it doesn't seem to be going anywhere.

When your client files MFS but not living apart from the other spouse, PTO will not flow the spouse's earned income to F.2441.  The only way to "force" that on Line 19 (which is not coded as an override - so be careful to update that should the filing status changes or when the spouse's earned income is altered subsequently) is to enter the amount on the line Earned income [Adjust] under the SPOUSE column, which is found under Credits > Dependent Care Credit (2441) > Children, Dependents and accessible only by clicking on General Information in the top-right corner.

As I said, PTO does not understand the application of DCB for MFS; it will produce an informational diagnostic that your client does not qualify for dependent care credit.  Simply check the box to ignore it provided you have checked that everything is correct and you will be able to e-file.