Keep the same size frame a run a 50mm stem with riser drop bars, if you need more stack height. Gravel and CX bikes are not road bikes and should not be subject to the same aesthetic judgement you'd get with running a short stem on a road bike.
The long running myth that short stems cause "twitchy" handling are just myths supported by bike publications that keep repeating the same ridiculous argument that holding your bars next to the stem simulates having a short stem. The only thing that simulates is having really short handlebars.
I've designed and built a number of Gravel frames with "forward" geometry that run 50mm stems and they handle just fine. The forward geometry concept of stretching the front end and using a short stem is very similar to sizing up on a conventional frame and using a short stem. If he is using an older MTB that was designed around a 100-120mm stem, he can easily go with a 35 to 50mm stem and drop bars.