How sweet of him! I'd go out and buy a MP base and make him some soap with those colorants as a reward for taking an interest and supporting your passion! You may find you actually like doing MP. It's a great option for making soap with kids and grandkids, by the way. (Or maybe you already have tried MP in the past. You've been around longer than I.)
Regarding the lye stability question:
I have no experience with this brand, which I believe has an "i" in it that you may have left out, but they do have a "contact us" link on their page. Perhaps they can answer your question, but they are clearly made for use in MP, so it seems unlikely they are lye stable.
Non-bleeding transparent colorants ZENICOLOR 5
Back in 2015, ZeniColors were exclusive to the Soap Kitchen in the UK and they do still sell them, so maybe you can find more answers from soapmakers in the UK or by contacting someone at the Soap Kitchen. Or at least you can read the documentation they have available as links on their site.
Shop our range of Zenicolor dyes. Bleed free and they maintain complete transparency in clear soaps - The Soap Kitchen™ Leading UK & EU Soap Supplier.
Zenicolor 5 for Melt and Pour Soap available from The Soap Kitchen
For queries go to this page to find the contact us information, if you wish to ask if anyone they know of has use ZeniColors in CP soap and what their experience was. About Us
But hopefully someone here may have used these in CP and can share their own first hand experience.
The only other mention of ZeniColors I found on the MP forum here at SMF, the two UK soapers who used them have not been online here since last October, so they probably will not see your query. But you could send them each a private message (via conversations) and see if they respond at some point. This is the other thread about ZeniColors where I found them: Melt and Pour Colours
Good luck with your research and I hope you find some useful answers!