Thanks guys - some additional information:
The other 'critical' program from their WinXP boxes (2 Engineers, one with PC issues), is ICPR v3 - this is installed, and works much better on their newer Win8.1/10 machines, but no longer supports RDP access as a result of Streamline electing to not update the drivers/software to work with the hardware dongle. RDP is still functional in WinXP, and since it's hardware locked, if/when they need to remote in they usually just move the dongle to the WinXP box.
I obtained a quote for the new version of ICPR that does support remote access, etc. but the owners decided not to purchase and use what we have. Another option I'm exploring for this specifically, is something like USB Director
Back to MODRET, I've already setup a WinXP SP3 Hyper-V VM for him (the Engineer mentioned in the OP); all he has to do is double click the shortcut on his desktop, and he's working in this VM.
He's just tried the VM adaptation for the first time today, and MODRET does work to a point, but then throws a 'floating point error'. The resultant report is lacking some of the data that is needed for that particular project.
Hence, my asking for alternatives, as these tasks are clearly done today by many other civil/drainage Engineers... I just need to know what tool(s) they're using, so I can offer him an informed recommendation (as this is out of my area of expertise).
I'm still researching a fix, as that would be the simplest thing. I have not heard back from MODRET's manufacturer.