Giving the odd spent grain treat to a cat won't harm it, but honestly even with tuna added, most cats simply won't like them due to them not being meaty enough. Treats generally have to be really meaty (or high in fake-meat flavour like Temptations, yuck) for cats to like them.
Of course there are exceptions, some cats love everything. My old cat loved having a tiny piece of the dog's cookies. But for cats you don't know and are giving as gifts? I can almost guarantee you it's a waste of time. And if the owners are savvy enough to know that cats are obligate carnivores vs. dogs who are opportunistic carnivores, then they may not want to feed them anyway.
My dog, however, would like you to send spent grain dog treats our way, because he claims the huge bin of spent grain biscuits I made him just aren't enough.