It is not uncommon to define a segment separator
like "|" is the X12 ISA segment (ISA16, character 106 of the ISA segment). Have a look at a related tutorial.
To my knowledge, characters with ASCII codes below 128
(hex 0x80) are allowed.
If your EdiInvoice
service cannot handle partner-specific segment separators, you most likely have to contact the developer of your tool or the provider of your service first.
As suggested by eppye: If the sending partner can switch to an "easier" segment separator, this would also be an option, but there has to be a good reason for the partner to invest time and effort.
If the syntax of the EDI 210 X12 message conforms to the specification, the sending partner has no obligation to change anything.