when i was at university I went out with a guy who couldn’t swim. over the course of a few months (and many trips to a variety of different swimming pools) we made progress. The first time we went to a pool he found it difficult to stand in deep water without feeling nervous but a few months later we could even manage going to a proper grown-up swimming pool with a proper DEEP deep end and lane dividers for serious swimmers!
best of luck with this! It’s sure to be extremely rewarding for both you and the (soon to be non) non-swimmer!