what brands and flavors is he using?
Is he adding sweetener to his juice? If so, well just don't
most flavorings are sweet enough on their own and *IF* after you're (he's) done mixing and vapes it and feels that it's not sweet, you can always add a few drops of sweetener to the mix afterwards until it's where you (he) likes it.
But most fruits and custards are sweet enough really on their own or combined together...I rarely, next to never use sweetener. Also because sweetener will gunk up your coils faster.
It might also be a using too much of the flavoring, some flavorings really only need a little bit to impart it's flavor.
as far as recipes go, would need to know more about what he likes to vape or what he likes to eat/drink
one of the folks here in the forums likes to add tfa acai or inw rhubarb to her vapes to help cut some of the sweetness out of them.
maybe instead of using custards try using a cheesecake flavor tfa cheesecake to me is less sweet or pur new york cheesecake, they work well for a cream or their intended purpose of cheesecake (but for me cheesecakes need more graham cracker and/or biscuit (cookie) for a base and mascarpone cheese to it...but I do like a more tart cheesy cheesecake)
I flip back and forth between sweet and savoury vapes
but yeah just a point of clarification
it's vaping/vape(s) there's no smoke/smoking a vape unless you over taxed the battery and it vents or you really really really burnt the heck out of the wick