Bit have you ever tried the Slide Show program in Windows 10? I have no other suggestions. Has the client used Roxio programs like Video wave?
Yes, the Corel photo editing program seems to be complex but there are "automatic" settings to make it easier.
Pinnacle has SmartMovie as their slide show offering.
At one time you could download the PhotoShows from the web and save them. I don't know if that is still available. As with other software, if Flash goes away, your client will need to get other current software anyway.
Yvette, did you try FastFlicks or any other video editing program to make your slide shows? Google for other software.
Rizzo, get Yvette, BitGuru, his client and a whole lot others to see what can be done to resurrect PhotoShow by contacting Roxio/Corel. It would have to be a whole new program though.